I can see how displacement may be an issue if you were building a Drag car and you wanted to produce 1200 HP. But then again that is far different than modifying a daily driver. I still think the Ford V10 has some appeal in that market. Then again, you have to build a motor to withstand the use of nitro methane, which means you cannot use production stock motors for that purpose. That is one reason you do not see any radiators on these motors since the block and heads do not have water passages.
Actually I have my eye on that honda 4 stroke single piston motor on my lawn mower. Just think of the gas milage I would get with that baby under my hood.

I wonder how many of those I would have to join together to even come close the the amount of power my 4V -4.6L provides?

Have you ever seen lawnmower races :thumbsup That is too funny , but those things can move.
I see no bennifits in crossing the line from one manufacturer to another. That would actually cost more.