Hi Gaz, welcome to the forum.
I think I can help with a couple of your points as I faced similar dilemmas.
1 - I'm no fan of the rear end of the bike - Yamaha really took their eye off the ball here.
Apart from the whole tail unit being way too bulky (which is a common curse with stock bikes), they've clumsily added retro-touches that don't quite work with the rest of the design (which I feel should be viewed more as a retro-futuristic-industrial hybrid), such as the tail-light taken off another, more genuinely retro model and bizarrely adding a little upward curl to the lip of the rear fender, which strikes me as very gauche.
Before the rear fender really started to bother me, I was also going to go for the Evotech Performance tail tidy, which is by far the best looking of the products in its category (imho of course).
However, since deciding to do away with the whole tail unit, the Evotech Performance part can no longer be used and the suitable options increase and became rather more complicated.
First, does one keep the original tail light or go with a different design?
While no massive fan of the original light, Yamaha have since released a more refined after-market version ([FONT="]LED Vintech Tail Light - P/N: YME-FYTL1-00-0).
Granted, it's not perfect but I do feel that it works better than the original and presents much more of a 'future-retro' vibe due to the halo effect, which will nicely compliment an LED halo headlight (the best of which is the pricey VisionX Vector, for those that are interested).
You may have already seen the thread on here about the Vintech but here are a few more posts and more importantly photos of the light fitted to XSR900s, from a German forum:
However, if you've completely been turned off the idea of round tail-lights and want an after-market one in a different shape, then things get a bit more complicated. (as well as the fact that despite having now looked at thousands of tail-light designs, I have yet to find one that I feel would perfectly work with the XSR700).
Turning first to the R&G tail tidy. With apologies to those who have fitted one, I just don't think this part looks right at all.
It fits way too high, with the license plate starting just a few cms before the end of the bike frame. It also has very little rear-ward projection, which adds to the small, bunched-up look.
The tail-light they fit also doesn't work at all with the design language of the rest of the bike and it fits in a weird position, not close enough or far away enough from the frame.
Worse is that it tilts upwards (though this is not uncommon with the tail tidies featuring after-market tail-lights) and just looks pretty cheap and 'Chinese' in the process.
I think there are better options to consider here but which still have their own flaws;
Urbano Bruni 'Racer Line' (still fits too high and with little projection although the tail-light at least integrates better)
Puig (too high, light again tilts upwards but is a little better than R&G one and fits more slickly. It would likely be easier to replace with another version too. However this unit does not appear to include a complete under-seat plate with which to cover the hole left by the removal of the fender)
Rizoma 'Fox' (usual ridiculous Rizoma over-pricing, fits too high, although at least it has adjustable projection. Light seems sleek but I'm not sure I trust these currently-trendy tiny units to be as visible as physically larger ones)
At one time I had seriously considered each of these before deciding on the unit I just purchased (today in fact).
I went with a lesser-seen option (in fact I don't think I've read anything about it on here), which is to be used with the Vintech tail-light.
It's from Evotech RL (an Italian company, not to be confused with the British 'Evotech Performance').
I chose this as it appears to have a nice amount of projection away from the bike frame and a nice downward curve overall, thanks to the joint in the middle.
This allows for the license plate to be set at a nicer height than normal (closer to stock) as well getting away from the abrupt, linear style of virtually all of the competing units (which makes them look rather home-made, imho).
It also lets you continue using the original indicators (courtesy of the cheap brackets they sell for this purpose) while deciding which after-market ones to buy (my current position).
I also prefer the way the tail-light is mounted to the unit, which would allow for changing over to an after-market one, should a suitable candidate be found, by simply making up a simple bracket to accommodate regular tail-light fixing positions.
The downsides of the unit are that it's pricey (I paid EU €193, with the optional license plate light and reflector as well as the afore-mentioned original indicator brackets and shipping to the UK) and that it also might appear too industrial for some.
Assuming none of those take your fancy, here are a couple of other noteworthy units to consider, if using the original/Vintech light:
Urbano Bruni 'Y Line' (the upward sweep of it may not work well in person and it's a bit tacky looking, what with the 'XSR700' cut out)
Barracuda (a bit odd looking, and not loving the holes plus the indicator mounting looks cheap and the joint in the middle is unpainted)
4 - I have gone for the XSR900 bucket solution, for now (with plans to fit a VisionX Vector LED Halo headlight later, perhaps with a different housing).
IIRC though, the modification required to fit this was pretty trivial, with just a small hole needing to be cut out (a 2 minute job with a drill and appropriately-sized small hole saw) to accommodate the headlights bulb height adjustment screw.
I hope that helps a bit.