In principle, 1.8m dish can not perform as 2.4m one, it is against physics. 1.8m antenna can only perform as 1.8m antenna. Startling from 1.8m it really does not matter offset or PFA, more important is used proper feed..... that is IMHO.
A 1.8m dish will probably be at least 1.5db LESS gain than a 2.4m dish. Plus, it'll "see" more of the satellite arc, which means sats might interfere with each other on certain transponders.