I have 30" SuperATV Intimidators on 4" Portals with a 30% reduction. I haven't done an accurate GPS speed check and the speedo is off since the gear reduction; I would say the speed is noticeably slower and definitely lost top end, that said, I'm still completely happy with the greater torque in the mud, where it had problems stock gearing with stock tires.
There is more maintenance and you have to be careful with the weak steering parts but those are easily fixed with an aftermarket billet steering box and beefed up steering arms. Portals turn a Honda Pioneer into a completely different machine. It's not a glorified Mule or golf cart with Portals installed, it even sounds different with the gears added! It's an animal and it gets me where I want with monstrous mud buggies tearing up our trails with BFG 54" mudder tires, full sized tractor tires or military 6X trucks making water filled ruts that will swallow any stock machine.< period!
They are also not for rock climbers or tight hiking trails, you're really wide with Portals, a plus in the mud environment.
Just a friendly warning: many members here will attack you for daring to install Portals! LOL