My V-10 EX is a dedicated tow rig for an 11K TT, I run 5Star tunes al the time, mostly the 89 Octane Performance tune while towing. I sprung for the Banks headers and Y pipe, Aeroturbine 3030XL muffler, 4.88 gears and 35" tire's (effective ratio of 4.39). I love the tunes (87 Economy, 87 Tow, 87 Perf, 89 Tow and 89 Perf) I like the Perf tunes for my towing need's better than the actual Tow tunes, they are a better match for my driving style. Have not run with the stock tune in years. For a hard working V-10 gears are the biggest performance mod you can do (short of a custom turbo/super charger setup) and good custom tunes are next on the performance list.

Crawl under your EX and take good look at the factory "T" pipe, yeah you want an aftermarket "Y" pipe! If you do the Gibson shorties and leave that necked down abomination of a factory Y pipe under there I'm not sure that you would feel the improvement from the headers, it's that bad.
You most likely have the more common 3.73 gears and that 3V F-450 probably has 4.88 gears, big difference and like I said above gears are the single biggest performance improvement for the 6.8. What does your door sticker say below the "Axle" header, should show a letter and number code, that will tell us the gear ratio it left the factory with and if it's a Limited Slip differential.

I think that '01 was the last year for the rams horn style Y pipe, after that they went to the very restrictive T pipe to resolve an exhaust flutter sound that some folks heard. So your style is the original and the abomination is the "upgrade"! The rams horn is obviously better flow and performance wise than the T, owners who have upgraded from the T to a full flow smooth Y report a noticeable improvement in throttle response and a bit better umph at higher RPMs, rams horn owners who have gone with a full Y haven't seen as much improvement as the first group.
I have the 5Star 87 Economy tune and really didn't see much better MPG from it, it will get you into OD more quickly under even very light throttle but I think any improvement brought was within the +/- of my normal tank MPGs. Strange as it may seem, my best highway mileage after all of my mods, some of which are MPG decreasers came on the 89 Performance tune, 14.5 MPG at 65 MPH over 490 miles unloaded with 3 adults onboard and the AC on for over half the trip.
How tall, long and heavy of a TT will you be towing? My best towing mileage has been 9.5 MPG over 420 miles pulling a 12' tall, 41' long 11,300lb TT from Niagara Falls back home to the Philly burbs. Several other trips have been over 9 also, my goal is to eventually hit 10 MPG. I'll have a good chance to do that next spring on a nice flat coastal route to FL. I think with a shorter and lighter TT, the right gears and tunes that regular 10 MPG towing trips would be possible.
And I agree that every MPG counts, so much so that I make a hobby and game out of the idea, my warm weather daily driver is a '94 Geo Metro that I've hit several 63+ MPG tanks with so far, next Summer with a few more mods I'm shooting to get a 70 MPG tank.

skip the 4.30’s and go to 4.56’s. With larger tires you will be happy.