That's an enviable goal mate, I wish I could get away with keeping all my cars, but my current 'fleet' of MX5 and ATR is serving me quite well, even if the ATR is predominantly the wifes car!
I think the issue between the 1.8 and 1.6 is similar to the B16B and B18C, if you increase the capacity without changing the head or cams, then the engine is trying to breathe more air through the same size apature, which is obviously going to strangle it a bit more at higher revs! (As is usual, I have a method of modelling this relationship for the B series...
) Seems it can be resolved very easily on the 1.8s with the Exhintake cam swap though, which is what I suspect mine may have had done.
Fuel consumption ain't great TBH, I'm getting about 25mpg where the ATR will do 27s, you can blame an inefficient, low compression engine, primative ECU and shocking drag co-efficient for that I think. Luckily my mate with a SC'd one said the fuel consumption didn't suffer that much when he did his, he just got a bit more for his money!