Top gear is pretty much the least of your worries.

Your top gear is for two things: sprints and fast descents. By definition, your top gear is NOT for speeds that you can sustain on the flats, even with a tailwind, because you always want some gears taller than that.

If you're not racing, then you're just sprinting for fun, so if you have to pedal 150 rpm to beat your friends to the city limit sign, that's good for you. When you're ready for a race bike, then you can worry about a taller top gear.

As an example (and I hope I've got my arithmetic right this time) - with a gear of 42x11 - a common mountain bike top gear - with 26" wheels, at 90 rpm you'll be traveling (very roughly) 26 mph.

I'm not saying that top gear isn't a consideration at all ... with that 42x11 you'll spin out very quickly on the descents, and that'll give you less momentum for the next climb.

But I do think it's a very small consideration.