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I believe you should maintain a diameter that is as close as possible to the original so that the gearing of the drive train is not adversely impacted. (speedo reading incorrect) From what I read on the web, a difference in diameter +/- 2.5% is a general rule.
Cups with tiny tyres get to 60...
As you are one of the CS billionaires, I don't suppose holding value is important to you. Your admission of wanting yourself or the car to be noticed strikes me as being the winning factor here.
Go for the v6, just in case some or all of the above isn't 100% factual. ;)