I switched to longtubes over shorties, ported the Performer lower, and put a new 306 in and gained 1.3 mph at the track.
I do not contribute the majority of those gains to the longtubes.
Since (when you race) you are in the mid-upper rpms, the longtubes will not help you much in my opinion.
The only real positive I got for longtubes is they sound better (more performance sound) over shorties.
1 5/8th inch headers (shorty or not) flow plenty for a stock engine for sure, even h/c/i.
Longtube Pros: Better sound, slightly better torque at the bottom-mid half.
Longtube Cons: Non-emissions, harder to install, less ground clearance, more expensive, have to buy a matching mid-pipe, watch out for clutch cables (burning).
Shorty Pros: Cheaper, easier to install, more emissions compliant, relative same power in the racing rpm.
Shorty Cons: You are no longer in the 'bigger is better' crowd
Either way, in whatever you choose, you will get a nice sound! Longtubes have the better of the two in my opinion.