IMHO, the Thunderjet/Thunderslide kits are just too rich for a stock engine with only intake/exhaust mods. Everyone I know who went that route had their gas mileage greatly reduced. Several also have plug fouling problems.
All I did on my Dyna was go up 1 jet size. I average 45 mpg, range from 40 to 52 depending on conditions and load. My needle was already on the rich side from the factory (they recalled the 2002 Dynas for this, I didn't let them do it...). Lots of guys are very happy with just replacing the stock needle for the 1988 Sportster needle.