I have a 97 k1500 suburban with a 5.7. the PO punched out the cats so i have a p0430 and p0420 code. im only getting 10mpg out of it. Do you think putting new cats on would improve mpgs. thanks for any info

10mpg sounds a little low for what you have. I used to have a '94 that would get about 13 in town and around 15 hwy, depending on load and wind. The backpressure will cause the truck to run better, with the new cats on. I would replace the cats and install a free flowing muffler, also have your fuel injection system cleaned at a dealer and it will run better, as well as yield better mpg's.

it has dual flowmasters. I had a 99 tahoe that got around 16 so i didnt think the lift and the tires should kill mpg that bad thanks

Do you even have an O2 sensor behind the CC?
I don't believe my 99 5.7 does.
I got O2 codes before and just went to the local jobber bought a couple and changed them.
I'm pretty sure mine only has an O2 sensor on each exhaust just below the manifold.

Anything OBD II will have an O2 before and after the Cat (roughly '96 and up). GM trucks with the 'R' engine 5.7l '96 and up will have 4 O2 sensors. 2 on each bank, 1 before and 1 after each cat.

As for increasing fuel milage with putting the cats back on, it might only because the systems will now be working all within it's peramiters, but I wouldn't expect much. I would look into air filters, fuel filters, and tune up stuff since you may not know the last time all of those have been changed. If the PO didn't put Delco Platinum or Iridium plugs back in, that could effect the milage too.