I did Pontiac G8 241 heads (upgraded valves and springs), longtube headers (with cats), custom exhaust, e-fans, and tune so far. I'm about the same MPG (slightly better/more consistent highway MPG) but it's a ton more fun to drive. Luckily I didn't shave the 241's down as next up is the CTS-V LSA blower and LSA cam. Now that will wake this big pig up. Should have about $1500 in the blower setup collecting both new and used parts, plus a new tune and hope to install late January/Early February. Aiming for around 425hp (stock is 285), 475tq.
Did you lose any power down low with that head swap? I have a set of heads off my old GTO sitting in the garage, just never really considered putting them on the truck because they were intended to build power higher up in the RPMs. My Caddy weighs somewhere in the neighborhood of 3 tons so I need all the low end I can get.