I don't have an 8800GTS 512, but I do have that cooler (lapped even ) on my 8800GTX and I can tell you that I got almost no headroom overclocking. of course this isn't a fair comparison as the 8800GTS uses a single slot cooler and my 8800GTX had a double slot cooler. I leave the decision up to you but I can tell you that my 8800GTX is much quieter now. Load temp is 53c idles at 44c. DeltaT is ~10c according to rivatuner (probably not correct DeltaT but, whatever)
They're fairly secure, I assume you worry about them falling off. They will not. Load and idle before and after:
Before (ambient ~27c)
Keep in mind this was with some crappy SilenX brand fan that had almost no static pressure. Well, i replaced that with a Zalman ZM-F3 (great fan, for a great price, sleeve bearing though ) and lapped my heatsink and they are now:
Thermal paste = MX-2
With an ambient of about 23c (its colder in my dorm). It was my first lapping attempt so i might not have done perfect on it, might fix that when I get my lapping kit next month , I'll try and take pics too.
dont know about GTS temps, be surprised if stock is higher than GT,
these things are fantastic, my stock was 53/85,
got a 92mm zaward led golf fan on mine & temps are 35/46.
overclocked now to 700/1750/950
the GTS does have a better heatsink/fan but the GT is terrible, dead loud fan & even on 100% the temps still get quite high!
i've got one on mine & dont regret it for a second!
Overclocking potential & great temps.
what more can you say