Originally Posted by wstar

I'm sure a good dyno tune shop can account for these with good software. What I recall from my LS1 days is that an oversized (vs stock) throttle body will tend to lean out the engine without the ECU knowing about it, so it can be potentially dangerous without a retune.

The MAF on the LS1 was calibrated for a given TB diameter. If you oversize without adjusting the ECU (or somehow adjusting the MAF output), the MAF output is actually wrong, and it's giving bad data to the ECU. I don't know if the ECU's data from the header-mounted wideband O2's can help it adapt or not, but I wouldn't want to be the first to try without knowing more.

Edit: Actually I'm pretty sure it would be a bad idea without a tune. The widebands might tune for it at idle or cruising throttle, but IIRC at WOT the ECU is going to ignore those inputs and use a preconfigured map, which will run the car too lean given the bad MAF input.