For what it's worth...
Footpeg relocation kit, does give more leg room and reduced my leg cramp on longer rides.
Hand protectors, wouldn't fit on my GS as hit the screen. Also, mountings suspect and not very strong if you have the heated grip option. I have an unused set in my garage as a consequence... I would suggest dumping the OE heated grips and replacing with some of the new Oxford ones, fit some renthal alloy bars, and then mount the hand protectors with the proper non-heated grip fittings.
Sump guard, I fitted the stock PD BMW sump and centrestand guards from Motorworks.
WIRTH progressive springs, instead I paid Ron Williams at maxton £195 for custom made fork internals. Well recommended conversion.
Oil cooler thermostat, if you have a cooler fitted to the bike already and ride in cold weaather at all, probably justified. It's a nice, expensive bit of kit.
Obviously a WP, Ohlins or other quality shock is essential as the stock unit will fail at some point. Flip-a-lever cruise control and 'fat foot' from schneider are great little gadgets. Motobins 4 pot front brake caliper with braided hose. Recon Bosch starter motor.
God I'm such an anorak.