I don't really know much about import shops around town, or any Z-specific ones either, so I can't help too much. But I can say I went in for Dyno runs at both
Carboy and
EngineLogics, and they both seem to work on imports, and they both seemed like quality shops. You might give them a call and talk to them about the kinds of work you want done and see what they say. Both are on the SW side of town, roughly in the vicinity of the intersection of Beltway 8 and 59S.