is there any loss in performance if the exaust pipes are not mandrel bent? also what are some good shorty headers?

yes, and check the sticky out at the top of the page. I recommend either the SLP or hooker headers.

I realize that mandrel bent is better, how much of a perfromance loss is there if it isn't mandrel bent?

It depends on the size of the piping and how much power you are pushing. A weak motor won't suffer as much as a stout one given the same pipe size.

I am thinking 3" pipe for (what will become) a mild/moderately built motor

Only problem is finding someone with a mandrel bender. I have found this to be quite the dilemma. Yea, I still haven't found anyone to fab up a mandrel bent exhaust after a year and a half, that's relatively in close proximity. go me.

Flowmaster, Dynomax, and Mac all make mandrel bent cat backs for these cars. I just put a Flowmaster American Thunder system on my car. Never dynoed it but just by the seat of the pants feel, I can feel an increase in performance. The old system must have really corked the car up cause its got a ton of torque now.

I thought dynomax was only 2 1/2" though? anyway how bad do headers really leak?

I run a set of SLP headers on my 350 motor. Extremely happy with the fit and the quality of the headers. Its a y pipe design is the only drawback. But the performance is there and i've never had any problems with the headers leaking or the bolts backing out. The installation kit isnt really worth the money but you have to buy it. I bought a few in bulk figuring i would need to repair it one day. Its been 3 years and no problems yet.