Over the last few years we've taken an essentially stock R33 GTST to Winton and gradually upgraded it, noting the laptime benefits of each upgrade.
We reduced the weight significantly, (probably around 200kgs removed), ran the same boost pressure and stock brake system during this period. We also installed a VideoVBOX system to properly understand what was happening.
Whilst this car had previously had Tein Street Springs installed it still had some pretty rubbish standard shocks. So no coil-overs...
Biggest 'feel' benefit was a proper fixed back race seat. Not a lot of lap time here, but major gains to help learn what the car was doing.
Sway bars: -2.1" per lap
Whiteline Blade Adjustable, as most people attest, gave a massive laptime improvement on the same rubber.
Brake reconditioning: -2.8" per lap
Still the stock brake rotors, re-machined, Ferodo pads, Dot 5 fluid and new seals all-round.
Semi-Slicks (Toyo R888's) -5.58" per lap
We also shimmed up the diff a little before bolting these tyres on so it's difficult to tell how much of which was of benefit, but I can tell you the diff shims were very cheap and the tyres, not so much!
There's a million other little upgrades on there, like a HICAS lockbar and undertray, rear wing, but there's no way for us to directly prove the benefits using the data... and asides from the lockbar they aren't 'suspension' related.
We will have BC ER Coilovers on soon, but also a completely new RB30DET good for well over 500bhp and a roll cage, so it won't be a fair comparison. But I hope this helps.
Some more details on the project including on-boards etc here:
http://droppingthehammer.wordpress.com/
Best of luck, hope to see you there soon.