Power restriction

Hey all, long time no see. Last time I posted I had oil problems, that was almost last December. I'm running a 1996 SC2 to refresh your memories. Well all the burning of oil caused a vapor pressure build-up that eventually BLEW UP my exhaust (including my Cat), I had no money for the new Cat and repairs so I told the guy to just bore out the Cat and I'd run it without it, he said that would work until inspection time and he replaced the rear exhaust for me - everything was ok for awhile. A short while!
Soon enough the oil problem was back, filling the 4th cylinder (passenger-side) plug with oil after high RPM's or long running time. Between that and the horrid gas mileage, my father and I thought we might as well learn from this so we completely rebuilt the engine in our garage over the course of the summer.
Well here I am, all new parts, almost completely new engine, $775 later and it runs the exact same (only after putting a custom-made RightStuff sealant on the valve cover I don't have an oil leak in the plugs that I've seen recently).
It starts fine, runs fine, for awhile. I drive about 20 miles to work each day and by the time I get off the expressway it can't 'accelerate' back up to 35 from the light at the exit (it gets up there, it just takes a loooong time and does so at the same acceleration rate regardless of how much gas I give it).
Another thing to note, if I stop anywhere (Blockbuster, for example to rent a movie) and come out within twenty minutes and start it back up, it BARELY starts and the RPMs are around 500.
So my Dad (who works for an automotive production plant) asks around work for any last ditch ideas, and irony of ironies, a guy says its probably because I have no Cat and it's causing a backpressure problem that makes it impossible to push air out any faster than a certain rate.
Is this really possible??
I would think that the fact that it can't accelerate would be under the hood somewhere, not under the actual car?!?!
Any more ideas or help would be very appreciated. And if the Cat/Exhaust is STILL the problem, anyone know a cheaper way or place to get ahold of one (Cat or Cat & Exhaust), since Saturn (thanks alot!) requires California State Emissions controlled devices on their Cat's so I can't buy a used one from basically anywhere.
Sorry for the long post but I'm about fed up with Saturn up to this point and I just need a car to get me to my job so I can buy a new car :p

since Saturn (thanks alot!) requires California State Emissions controlled devices on their Cat's so I can't buy a used one from basically anywhere.

Emission control devices are federally manidated, and buying a used catalyic converter is illegal no matter how you look at it. that shop could even be fined by the EPA if they knew they hollowed out a cat. saturn isn't to blame for this.

anyway, back to the problem. if you disconnect the downpipe from the manifold and run the engine(don't drive it this way, bad idea) and power is to par, then reinspect the exhaust for resrictions. no cat can negitively effect power, but that doesn't quite sound like the problem here to me. i have known bad converters to takedown muffers with them. check all the tune up items - plugs/wires/air filter/fuel filter. also before driving make sure the dipstick is fully seated, after drive check it, has it popped up at all? when you rebuilt, are you sure the Cams where timed right? to be honest with you, this is one of those problems i would need to see for my self.

I had a clogged cat on my 88 c-1500 and it cause a huge power loss, had hard acceleration, and had a hard time starting after running for a while. I bought a universal cat from a local auto parts store for around $75. It cleared the problem right up. What I was told was the cat deteriorated after a while because antifreeze from the blown engine that was in it and a piece of the catalyst fell off clogging the exhaust pipe. The problem starting was caused by too much backpressure from exhaust gas build-up. I don't know why your having the same problem because yours was bored out. It could be that he didn't gut the cat but just ran a pipe through the ends and now a piece of catalyst is clogging the ends.

just a thought, the PCV valve also causes high oil use, lack of power, etc.
Remove the oil filler cap, and check for back pressure.