I might buy a 90 Camaro RS with a LO3. I have never owned a car with TBI, so I know zip about TBI systems. First off, are they a pain to work on? How would they compare to a CC carb system in this respect? Second, how do they do in cold weather? Are they hard to start when cold(like a carb)? And finally, how do they do gas mileage wise? TIA.

they run their best in cold weather and on startup. TBI loves the cold air. I dunno if its as cold here as there. CA

Everything about TBI is better than a CC carb. The cold start, gas milage, performance, all are better than most cc carb systems. Think of TBI as TPI but with 2 large injectors and a wet flow system (not dry flow manifold). TBI does everything TPI does but it's a wet flow design which has plenty of pros and cons. The dry flow is more sensitive to engine temp while wet flow usually runs better with a warm (190-210) motor.
Just remember that like a speed density EFI system (no MAF sensor), you NEED to make tuning adjustments to fuel pressure and timing or you'll have problems. The speed density efi has no way to measure the amount of air coming into the engine so if you put on lots of free flowing goodies make SURE you adjust fuel pressure to compensate for the increase in air. Or you'll run too lean an air/fuel ratio.

Would I need to adjust fuel pressure for something like a catback and headers? Those two things and maybe a cam swap would be about the extent of my mods for this car.

With the two or three things you would do to your tbi don't forget the free mods like the ultimate tbi mod, afpr(adjustable fuel pressure regulator). With these extra mods you'll be surprised at the performace you can get for next to nothing compared to going with a carb setup.

TBI is definetly much much easier to mess with than carb or TPI...the tbi will adjust itself to all climates and everything automatically as for a carb...yeah right...TPI is nice but is much more expensive to get high performance out of it and is harder to work with. i say go with a TBI car...may not be the fastest but they are reliable and can be made to be fast...fastest 305TBI that i know of so far ran a 13.1 with the stock cam and heads (id say low 12 easily with the heads and cam)

I usually think of a TBI more as a carb with two injectors instead of jets, fuel bowl, (and indeed the entire fuel metering circuit of a carb). Take away all the fuel related parts of a carb and what you have left is a throttle body, hence the name.

The TBI is much better in every way when comparing to a comparable CFM 2-barrel carbureter. When you bring in the big guns of carbs, then it's no longer appropriate to compare the two systems. Also, in TBIs, you have so parts to gum up and get stuck, and no fuel starvation problems on hard cornering, braking, acceleration, or while rock-climbing (TBIs are somewhat popular swap on older Jeep rock crawlers). I know you won't be doing much rock-climbing in an f-body, I was just pointing that out for my own amusement.

By the way, it does have a way of measuring the amount of incoming air. It's a function of the displacement of the motor, RPMs, MAP reading, and I believe IAT reading (so it knows the density of the air depending on temp). I'm not sure of it though, I leave the specifics of ECM hacking and function to the experts, but that's the way I believe it does it's business.

Still, that doesn't mean it knows what's best when you mod the motor... Just what's best for stock trim.

No. It doesn't have a way to measure the amount of incoming air. Key words are measure and amount. Speed density EFI systems have preset tables. Once you start doing your own eproms you'll notice that everything you change in the tables is a guess. Only way to actualy measure the air is with an accurate sensor like a MAF which uses the heated wire setup or something more exotic.
Classic example; stock Lo3 vs. a built 383 stroker. The Lo3 at 4000rpm and 80kpa will use WAY less air than a built 383 stroker at the same 4000rpm and 80kpa. Hence the no way for the computer to know the air SO you have to adjust with fuel pressure, injector size, base pulse width, VE tables, and AFR vs ___ charts. I would love to have a MAF system but the sensors are too expensive and then I wouldn't be able to run an open element

edit: If you would, understand when I said built 383 I'm talking about a larger cam with more lift, duration, etc. The cam, heads, all that jazz will flow more air yet at the same kpa it'll flow a larger volume of air.