to drp to a 2.8", theres alot that needs to be done, I would say the 3.ish rang would be safe, but find a used LS1M Scanner (or similar, they run about $100) so you can keep an eye on your *KR. I'm running a 3.4 right now, w. higher compression l36 block(.5:1 as opposed to a 8.5:1) and seeing a few degrees of knock (mostly tuning issues). Any thing under a 3.5 you will need a powerlog/PEMs/Headers, 180* T-Stat, tune and 1-2 ranger colder plugs. You wont nec. need to do a cam if you want to go further, yu can get a set of 90# LS1 Beehive(yellow) springs and 1.9 Rockers. once you go under a 3.4 pulley, you should prob look into either an Alky system or IC, hell even a SMALL NO2 shot (50) would help cool the intake charge enough to not need an IC. You'll prob get better performance out of the the Alky than the N02, and an intercooler than both.
Dont forget you will need a pcm whenever you start modding, if youre failiar w/ tuning, DHP offers excelent software for this ($500 new/$300-400 used).
With a 3.4 pulley (and supporting mods) you can expect runs around the low 13 range @ ~10xmph, depending on track/alt./temp/tires/driver/etc... IIRC, you're in the mid 14's now (stock).
your most restictive area of your exhaust is the downpipe & "U Bend". there are a few options to fix this, but a 3.6, dp, PEMS, 180* t-stat, plugs and tune should put you in the mid 13s