If you get rid of the waterheater looking muffler and go a highflow cat and an open air cleaner or CAI, you'll se a lot more gains from the Fastman TB. Also, if you buy a stock TB gasket and throw it on there, you'll need to trim it for the opened up TB, or your just choking it off again, plus you need to be careful when you line it up, as it can hang over into the air stream.
This is coming form somebody with a 5.2L that got a truck with Stock 3" exhaust piping. Once I got those restrictions out of there and opened up the TB a bit, I got much better results. I would think the 5.9L engine would be much better.

 

I have the 52mm fastman and it was THE best mod I have done. I was not looking for speed though. I tow with my truck and wanted it to tow easier. I did notice that the truck started quicker, ran smoother and had better exceleration after the fastman was installed. All of this was before I even fixed the intake manifold gasket. After the intake was fixed, the truck runs even better but my intentions were never to have a speed demon. If I wanted to make this truck go fast I would have just put a supper charger on it and left it at that. I have oversized tires but in the 30" range and the 3:55's. When towing I get about 12mpg, when empty I get about 15. At $4/gal of gas, going fast is the last thing I care about. I just want the truck to run as smooth and strong as possible and I believe the fastman did that. Jeff.

 

Mine just got delivered! I have to finish putting the new windows in my house before I can go play with the truck. I'll let you know the results.

 

I noticed alittle more response right off idle, thats about it.

 

Im going to be adding this soon as well.

As the others have stated,I am not expectiing it to be a supercharger type of noticeable gain,but only an addition to the other mods done already and will be doing in the near future.

I am also not looking at it for a speed upgrade,gas is too much money to have a lead foot.Only looking for a little better MPG,a little more acceleration and hp.

 

Well I just got done installing mine! And spent a 20 minutes writing a post to this thread about it and the upgrades I've done comparing upgrades and when I was about 5 minutes from being done the website screwed up AGAIN and all was lost! So whatever!