something i always wondered,,,wouldnt the xr,s last longer behind a twin step hull,,than a straight botom hull,,,seems like they would work harder pushing more hull displacement???

Swapping to the SCX upper is probably your best bang for your buck and should hold up to some decent power for a long time to come. 9 time out of ten it's the bravo uppers that go, the lowers tend to be a little more robust (and cheaper if they do go).

Selling you stock drives while they are still good offsets a decent chunk of the conversion cost.

Converting over to the Arneson will likely give you the most all around satisfaction (assuming you don't want to go crazy with the power)and speed, but everyone I've talked to has said it was more work and more labor cost than they had anticipated (sometimes partly filling the notch, relocating engine height, exhaust mods from moving the engines, etc)... but worth the effort in the end. It's not a cheap conversion though and at that point I'd seriously consider selling the boat and buying something that already has 6's or surface drives on it. Probably cheaper in the end and better resale IMO.

Correct. The notch has no relation to the drives on a side by side application.