4.88s would be ideal. I've got a raven topper on my T100, and by all means you can tell its there, even with my 3.4. The damn thing weighs close to 200 lbs. not much, but with our trucks, every pound adds up. I remember with my old '90 with a 3.0 you could feel a little more umph without the flimsy sheet metal topper I had on that.
I looked into a flip pack for my truck a while ago. Even went and looked at one. They are by no means light. The one for my truck outweightd my current topper by 75-100 lbs. I even test fitted it and you could see the truck visibly sag about a half inch. They are awesome, but you will notice it.
Here's the topper I'm running.