I don't understand why you have stayed with the 8V head for so long. Are there rule breaks for staying 8V vs 16V? You should be able to make an ITB'd ABF w/ ported head motor make some serious power on premium pump gas, race gas or e85. I know nothing of the FSI heads, but do know there currently is nothing aftermarket to support DI, and tuning or finding tuning for a NA DI motor will not be cheap, unless there is more support avail in europe, dunno.
And what is a "decent "full options" megasquirt system"? MS1, 2 or 3? I would go with the built ABF head and block and leave the FSI head for future development. Is there a cost effective diff in flow between a built ABF head and a built/converted FSI head? Being that the FSI head is a much newer design, it should be an improvement, and stock it probably is, but is there enough improvement after both are built to justify the cost? I doubt there are FSI heads available like the 16V heads or even the 20V heads. I know there are all kinds of cams for the 16V heads and a decent number for the 20V heads, I have no idea what is avail for the FSI heads.
And you ending questions are good, but I haven't even seen any builds on here to even begin to answer them. You might have to be the ones to venture into those waters and find out, but that sounds like time and money you may not have.