Excellent answer Circolo
Ignition timing being off is a major power sap for the 1-litre, I've found. Usually because of the vacuum advance capsule being leaky and the static timing being played with to compensate, usually making the idle rough as a bonus.
I don't like performance air filters myself, mainly because they usually don't filter as well as the standard types, and even on the Uno Turbo there's little difference. I'd say on the 45 the difference would be less than one horsepower

'Car Mechanics' magazine put a cone filter on a Nova a few years ago and actually noticed a tiny drop in power, within the error of the measurement process though!
It would be a different story if the carb(s) were bigger, cam a bit hotter etc. I reckon.
-Alex