When you say mid-throttle, do you mean mid-rpm? for me it's either idle-throttle or HOLYMOLY-throttle

Seriously now, what percentage nitro are you running? perhaps your plug is not really suited for that percentage nitro at high-rpm, so you have to lean it out to get acceptable high-rpm, sacrificing your mid-rpm? I dunno much about this really, I'm just throwing ideas out there. Perhaps try a plug one step hotter or one step colder and see if that helps.