I had a Borla system on my last TR, and it did definately improve the performance a bit, you could feel the difference as well as hear it. But I would estimate the improvement was small, maybe 15hp to 20hp, the engine did rev quicker than it did with the stock exhaust. Any system that reduces weight is good for the TR, since these are heavy cars and most of the weight is at the rear. Keeping the stock system, and just gutting out the precats and cats will get you a few ponies, as well as greatly reduce the heat in the engine bay. The TR's stock headers are pretty good and don't need changing. As for the precats, there are factory "precat-delete" pipes out there but they are EXPENSIVE! I would just gut out the stock ones, they flow decent as is.
Keep in mind, when a free-flowing exhaust is installed on a TR, you may need to adjust the fuel mixture slightly richer to compensate, or you could get a "lean" condition in certain situations. To do this properly you will need a 4-Gas Analyzer or take it to a shop so equiped. What ever system you use, you MUST have bungs for the O2 sensors, the TR's fuel management system needs that feedback. If you remove the cats, you can also remove the thermocouples and their ECUs - they won't be needed anymore.
I do agree with most - aftermarket exhaust systems for Ferraris are WAY overpriced for what you get. I think the Kreisiegg (not sure of spelling?) is the MOST expensive system on the market at around $9,000 or so - it's a complicated system that makes that "F-1" sound many people are after.