A lot of posts on this - no idea what you mean by a "Performance Shock"

A much better shock than stock is the Progressive 412

if you want the stock bike stance:

412-4232C for a soft ride @90/130lb springs
412-4233C for a firm ride @115/155lb springs

might want to search on these  

russ_g wrote on 12/31/69 at 16:00:05:

LOL, then you could basically just put some springs on it to fit that definition.

Seriously though, the 412's are a big improvement, the 418's would be even better and the 440's are the cushy upgrades.  Of course, unless you get a great eBay deal, there's a pretty big price difference too.  I have the 412's on mine and like them a lot.