I ride a mid level 2010 Tarmac with Ksyrium Elites. I trained hard (for me) and started road racing this year in M35+ 4/5 with reasonable novice results of not getting dropped and being around to contest the pack sprints without actually threatening to win. From my limited racing experience, upgraded wheels would not have made any meaningful improvement in my race results, what I need is to learn better race craft (don't underestimate this) and improve my:- 1 min, 30 sec and 5 sec power.

All that being said, a friend of mine was kind enough to lend me his older Al/carbon Zipp 404's recently and I used them for one fast group training ride. My experiences were: they seemed to spin more freely (better hubs or bearings or better adjusted?), they held a tighter line on sharp bends, jumps/sprints seemed about the same, very difficult to make any judgement on difference in top speed or power used to hold given speeds (typically in to 24 - 28 range). However, they looked fantastic and based on this alone I will probably end up getting a pair of deep carbon rims later this year, either used 404's or new November Rail 52's.