and I wonder if it is worth the extra money for the achieveable speed difference for an average club sailor, who is definately not a top flight F18 contender
No,
Such a sailor will not notice the speed difference. The difference is really small you must be sailing for the top 10 spots in a 100 boat fleet to notice it. For club races and fleets of 10-20 boats you'll end up 1st or 2nd anyway with sufficient skill. The difference is just that small. Besides most of it is due to age (old sails etc) anyway. That is something that even OD classes suffer from.
My advice and that truly does work best is to buy a relative cheap and well maintained F18 (probably an I-18) and sail/race it for a couple of season and ONLY when you skill has progressed to the level of a top sailors switch to one on the newer F18's. You'll get most fun out of ever single dollar and there is absolutely no point in spending more dollars then you have too, you really won't notice the difference. The difference between F18 and I-18 is magnified by some people. We could have the same argument about 4 year old tigers and new tigers even when they are the same design.
Work on you sailing skills first and on the make of your baot later.
Also take note that Carbon masts are not allowed on F18 boats ! I think a few of the I-18 were fitted with these.
Wouter