I still think it might work with a remap.
With carbs the story is always that you loose if you go too large -- you'll tend to loose out on pick up and bottom end, because carbs work with vacuum signal (and on more sophisticated carbs, an accelerator pump to help out when the signal is weak) and nothing more.
People are getting away with absurdly large ITBs on Zetecs (54mm!) without ill effects but to gain you need porting, valves, exhaust mani, cam to suit and a custom map. Somewhere in PPC Dave Walker does a write up (and looses a bet that the 54mm ITBs would be too big.)
The 16v 1.4 FIRE engines seem fine with a single 48mm Dell'Orto (ex K series) but I think J333EVO is running a slightly larger Ford TB. These are all with stock internals, but better exhaust manis and aftermarket ECUs (= remap).
It's strange that the SPI cars react so well to a 40mm TB, but they do!
Without a remap and a better exhaust mani I doubt you'd see much, although the P75, IIRC, uses a 38mm TB. (Exhaust mani is certainly better than the later MPI cars, although I suspect that the inlet mani isn't quite as good.)