Well even though Guy might get faster and stronger, we have to remember that Naruto in Sage Mode is on a whole other level as well.
And in his fight with Kisame, he did defeat him but not kill him (he probably could have but it doesn't matter). What i am saying that Guy's strongest attack (so far) wasn't enough to overpower Kisame and also kill him at the same time, but Kisame is just your regular shark ninja dude, he wasn't even fused with Samehada, basically he didn't have any kind of special defense, while Naruto in Sage mode on the other hand has increased resistance (as seen when fighting pain and when training for SM).
Both modes grant incredible power but are limited to the time they can be used, i think in the end it's all about who lasts longer... that or if Naruto hits Guy with a rasenshuriken, then it's all over xd