I just finished converting my son's '79 Spitfire (with catalytic converter) to a European twin SU HS4 carb setup with the matching intake and exhaust headers plus new head pipe. The total cost was about $100 cheaper than a Weber DGV setup, and the car still is British! The combination of the improved carb setup and exhaust setup, plus desmogging has made a very fine improvement to the car's performance.
Compared to the ZS carb, the car runs much nicer and cleaner. The ZS carb when properly set up (for a few days/weeks) gives very good gas mileage (up to 38 mpg on highway), but weak performance. The SUs still give decent gas mileage (>28), but it's hard to know how much, because my son has not taken a good trip in it yet, and he likes to use that performance he's got now! Still he seems to be getting about 28 - 30 mpg in mixed driving. The conversion without the exhaust stuff is pretty easy. I had to swap out the catalytic converter/head pipe, cut the exhaust pipe and make an adapter, but boy, what a difference. I would estimate that the engine now produces 20-25% better power. The European Spits make 71 horsepower with a 9:1 compression ratio. Our domestic Spits make (maybe) 53 horsepower with a 7.75:1 compression ratio. I've seen one dyno test where a '80 Spit engine made only 48 hp! Anyway my guess would be that my son's Spit now is comfortably in the 60's. After a carb and maybe exhaust upgrade (and desmogging), the next steps are more challenging - higher compression and a hotter cam - and more money!!!