Yep, dt's don't tune easy, the successful ones normally need a scratch built exhaust and sensitive tuning by someone who has done it before. Just opening up the ports will as so eloquently put "make it run like shit". Esp the exhaust leave that port alone.
For what its worth, around 50mpg is doable and as for top speed, if you run it to the red line you may touch 60mph, makes a 0-60 time irrelevant. As far as it matters go is not what the dt was for, riding gentle through dirt lanes and fields its difficult to beat for strength and low speed manners.
You want speed, buy an RD/TZR