Hp loss at top speed

My boat went in last year for maintenance due to carbon build up. The shop had it for one month and finally got it back to me . They basically replaced my entire fuel system on my VMAX 200EDFI. I took it out briefly after I got it back and it starts and runs fine.....I spent a small fortune to fix it . My problem is now I'm not getting my top speed that I was before it was fixed. Shop said everything checked out and boat was fine. I am not happy and am not going back to those clowns.
Any ideas as to how I would lose top end speed? I was able to go 50 52mph, now only like 45?

What rpm's are you getting at WOT now?

Do you really have an issue here? If the motor "starts and runs fine", can you not live with the loss of 5 mph? If not, take it back to the shop and demand that they fix it considering how much you spent there. I agree with aliboy- do you get the same RPMs now?