In addition to Geoff's comment about the smoke (and it WILL smoke heavily w/ the 70's), you're also going to notice a slight drop in fuel economy, plus you'll have to watch the temp gauge a little closer, too. Nature of the beast when being over-fueled w/o enough air.
BTW, have you weighed your coach? With and without the toad? What kind of weight are we talking about here that you're trying to push down the Interstate?
I may have mentioned this before, but the charter fleet I worked at back in the mid '80's had six coaches with powertrains similar to yours: Two MC-8s and two MC-9s with exactly the same set-up (8V71N / HT-754CR), and two MC-9s with 8V71T / HT754CRs. 275 hp for the natural engines, 350 hp for the turbos.
Yes, you could feel the difference the extra 75 hp provided, but on the 250 mile run to Disneyland from Fresno, it made all of about 3 - 4 minutes difference in the travel time, not to mention burning about 20 extra gallons RT in the process.
And the fleet's little 4106 was faster yet and burned almost 30 gallons less! With N60 injectors and only a 1900 rpm max governor setting! (Don knows the fleet - he's worked on their coaches since they have a garage in Bakersfield.)
So here's my nickel's worth for you to ponder:
1. Go for a ride in somebody else's 40-foot Eagle, MCI or Prevost w/ the same engine, so you can get a better idea of your bus's performance. It may be you're really just expecting too much - they're not BMW's you know!
2. Since the engine runs decently, have somebody who really knows what they're doing run the rack on it properly, including setting the jakes. Use Don's specs - I think Geoff will agree to the settings Don uses.
3. Once you've got the rack run right, drive the dickens out of it for now, taking your time to assemble all the various goodies to make it into an 8V71T. As TomC mentioned in an earlier post, his 8V had "transit" rings - something that was new to me - but either Don or Geoff can elaborate on their advantage when being used on a "smoke turbo" engine, which is basically what the 350 hp model is.
4. Instead of cutting off the hitch, consider a lighter toad. The most popular toad around my neck of the woods seems to be the Honda CR-V, followed by the Jeep Cherokee. There's a Prevost owner about 10 miles from me that pulls a 5-spd VW Jetta TDI wagon behind his bus - says he doesn't even know it's there (8V71N / HT-740), plus he can fuel both together at the same pump!
5. Don't get caught up too much in the "Tim Allen Syndrome" - gotta watch out for those Binford Specials! LOL!!
FWIW & HTH. . .