Having a VW, I am familiar with the "touchy" Solex carbs!
Check the carb float. I've had SEVERE performance problems that were remedied with a float adjustment.
The accelerator diaphram may be only slightly ruptured, just enough to manifest itself under hard acceleration...your symptoms sound like that.
Also check the carb jets for obstruction, and the housing for wear. Even if the case is not warped, the butterfly linkages could be worn beyond spec.
Also look for loose or missing parts. I lost a piece to a linkage that would cause sluggish performance (on a dual carb setup).
Just some things to consider...