The 4 cylinder easier to tune? Sure, but just because the bits exist. Piper do a set of cams for the kv6 that could be reground for some more lift and duration, if only there were some around used, bit pricey from new. Limitations on the Kv6 are around bore spacing/dimensions - the unit is very short, but that's not a bad thing in some ways.Don’t bother with a kv6, they aren’t very tuneable unless you’re happy to spend big money on them. The k series 4 cylinder turbo is much easier to tune…
Depends totally on your budget. If you’ve got plenty of cash I’d be looking at an Audi s4 / s5 3.0l tfsi. Good for 500bhp with a map and couple of supporting mods.
Otherwise you want a Nissan turbo lump.
Decide on your budget, then decide what power you want. That will identify (or not) what engines are suitable.
Not thought much about the Audi lumps, will do a search, but Again does anyone do bits for them?
Budgets change, especially on projects likely to go on for years, but it's a good approach.
A bit suprised no one has any experience on the Honda, not sure if I believe the bullsh**t on the American websites, but at least stuff is available for it, if not much in the way of evidence of performance.