Having the correct air filter should make a big improvement over none also check the airbox is sealed with no air leaks anywhere.
Mid range could be mixture where one jet in the carb overlaps the next, normally If its a smooth flat spot with no missing just a flat feeling I would say a lean spot.
Could also be something to do with the power valve tho as mid range revs is probably about the time the power valve starts to open. From memory the power valve is not a big mission to remove and check everything is correct as per the manual, as said before I know the RGV250 has a bad habit of dropping the valve causing it to hit the piston and making a big mess. May be a similar set up to the 150 so you could check out on line
Sounds like you are heading in the right direction if she’s running better, can be like banging your hard against a concrete wall sometimes, best thing I found is do research and learn as much about it as you can.
Also I’m only trying to give pointers so up to you to do what you will with the info and disclaimer time, if she self destructs its all on you!!! So don’t
me.
Cheers
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