I have similar system with you, gtx 980ti, i7 3770k and 16 gb ram.
no troubles with other modules which are even have more complicated systems...etc.
despite I changed my cockpit setting to medium, I get low fps.
on groud or on air it doesnt matter,mig-21 has the worst performance for me, it costs almost -10 or more fps.
I dont want to lower my all graphic settings for mig-21 only because all others works smooth.
I know I am not the only one here, I know alot of user keep their mig-21 in hangar until it will be fixed