This is exactly the problem. I get so many conflicting answers. Half of people I ask (such as this forum) say it will make you slower and less athletic, but others say it will actually do the opposite.
My fear is, that being I haven't worked out much in life (except maybe a year) and I eat like a slob, I still have large chest muscles (can flex them), biceps, shoulders, muscles on my forearms and such. The marine calculator I used which is based on weight, neck size, waist size, etc, says I'm 200 pounds of lean mass. Now once I enhance my diet and step up my weight training, I see myself hitting 240 and it mostly being muscle.
I can see myself being Kimbo Slice and honestly, he's pretty stiff when it comes to knees and leg kicks, things used in martial arts. He's also somewhat slow overall. I want to avoid that. I want all of the power of a big man but the speed of a little guy.
Let me add, right now, I have waaay more fat than I would like and even with the fat, I'm still pretty athletic. I can run, jump, do all kinds of things most people wouldn't expect when you say 20%+ body fat.