I would have put the external filters on the inlet side too. Might have been easier to just change the tank's inlet filter and be done with it. ??
Changing the Slide springs is a possibility. However, I am NOT the one to speak to the effects of the different springs. I have never changed these and tested them. However, ...
There is also needle clip position to consider. If you lowered the clip, the needle would be raised and more fuel could pass. I have recently read an article where they were even using shims on the clips to get discrete changes.
I guess I would start by setting the sled up on a stand and setting up the fuel level test. If you verify that the fuel level was NOT dropping at WOT, then considering other things like clips, bigger jets, etc would be reasonable. But, until you know for sure enough fuel is getting to the carb at WOT, I would have to be a bit skeptical of other changes.
Crap stuck in the needle/float valve could also skew a carb's performance.