For the most part, just about every emissions testing program I've seen is more about $$$$$$$$$$$ money than really cleaning the air up. In Missouri if your car doesn't pass and you go to a
"Recommended service repair garage" after you spend the required amount of money,if it still doesn't pass you get a waiver (meaning that it doesn't have to pass).
One woman that I was talking to at the Gateway Clean Air testing station told me her horror story.
She took her truck in and it failed, so she took it to the recommended service garage. They charged her $325 for what was supposed to be what sounds like a basic tune up (air filter, spark plugs, distributor cap & rotor), which is rediculiously high even for that. She took it back to the testing station and it failed again. She took it home and her husband looked at the truck. The service garage had put
absolutely no new parts on it, they had just charged for them. She went back to the garage to question them about this and demand her money back. They told her that it didn't matter if it passed because she had spent the required amount and the only way they would give her her money back was for her to give them back her receipt showing she had spent the money in an attempt to cure it's failing the emmisions test. No receipt, no waiver. The only thing they didn't know is that she had made a copy of the receipt.
The sad thing is this isn't the only story like this, it happens all the time.
The only reason there are any emmissions testing programs is that it's easier to go after John Q. Public than a major Corporation. The public will whine a little and live with it, corporations will move to a third world country and do their pulluting there.
If any of you really knew about the pollution the D.C. actually puts out you'd be shocked. If the EPA knew there would be major fines, but no one is going to risk their job to turn them in.
To answer the original question:
I took the cats off my 94 Viper (which BTW only has one oxygen sensor on each side so you don't need the sims) and there weren't any problems with it passing emmissions.
For that matter, they didn't even test it for one of the things they had failed another of my cars for weeks earlier (I think it Co2, but I'm not sure without digging for the paperwork).
As far a HP and torque gains it added about 10 to each of them.
There is no bad smell from the exhaust.
The only thing possibly bad is that at certain rpm ranges it does drone in the interior with the hardtop on.
And if you're only getting 10 mpg out of your's, there's something wrong with it. Driving mine hard the worst I've ever gotten is 15 mpg out it, but I usually average 22 mpg (a mix of hard and easy driving).