the first problem is just getting it to be high powered (its expensive..but can be done. turbo/supercharger/nos are the most obvious answers..engine swap would be more ambitious).
after you do have the power though, the limitations are basically:
1. fwd (as mentioned in the post above me).
2. weight distribution. most of the weight in the celica is in the front of the car (i forget the exact ratio..something along the lines of 60/40 or 65/35).
this might look simple, but it really isn't. because the celica is fwd, its good to have most of the weight in the front of the car because it improves traction (more weight on your pulling tires = more traction). but fwd is known to be bad for acceleration because when you stomp the gas..the lift (not in the context of vvtl-i..this is for any car) exerted will take a lot of the weight from the front of the car and virtually shift it to the rear of the car. because your 'taking off' a lot of this weight on the pulling (front) tires, your decreasing traction and too much power will just make you spin out (which is basically a waste of power).
but if you were to make your car a perfect 50/50 weight distribution..this wouldnt necessarily help it out much because when you slammed the gas it would still throw the weight (which is now about 20% less) from the front and shift it to the rear (non-pulling) tires...which hurts traction even more than before.
that said, if your wondering why 50/50 weight distribution is so important--its for handling reasons. a car with 50/50 weight distribution has a very narrow center of gravity and because of that...it handles like heaven (this is why almost ALL exotics are made with the mid-engine design..it helps center the weight). if you've ever tried making a quick lane change with your celica (or for basically any front engine car) it'll sway like crazy. this has some to do with suspension, but a lot of it has to do with the fact that the center of gravity in the celica (and most other front engined cars) is geared toward the front so the tail end of your car struggles to maintain a balance.
that said, it's obvious that you can't fix one w/out hurting the other..you just have to go for balance. if you are more geared towards drag racing, then you really don't need (or want) 50/50 weight distribution because (as mentioned) it will just hurt traction during acceleration). if you are more geared towards street racing or just want your celica to handle better..50/50 is more for you.
given enough money, you can convert the celica tranny into rwd...and that would be a very good place to be if you get into the 250 whp range.