I did talk to a guy selling a 26 Lavey. He had a 496HO with a procharger on it. Said it would run mid 70s, but wished he had done a 525.
I'd really love the Merc 600 or an Ilmor 625, but $40k for the motor plus the hardware to support it is a lot of $$$
Ultimately, it comes down to what is fast enough for you.
My wife is fine with 60, but much more, and she would be giving me "the look". Plus, anyone on my boat is usually fine at that speed, so I personally don't need any more.
So, if faster than high 60's in a 26 footer is what you want; go more than an HO. If you want to just cruise at 45-50 and top around 67-68; go with the HO and spend the money on other upgrades that you will use more. Tabs, billet, stereo, maybe steering? Just an idea.