While I agree with you on the Bf-109K one on one vs a P-51D you also have to remember though that the Bf-109K lost some of its great flying characteristics as it got faster but heavier (not as much as some people like to make it be, but it did). Now having said that, I voted for the Bf-109K as well.
Chris - I just happened to stumble on to a quote in Caldwell's JG26 Top Gun book. Uffz Georg Genth's comments pg 308 on the K-4 being supplied to III./JG26 in November, 1944. Won't quote the entire paragraph but to summarize.
He much preferred the G-10 as a dogfighter (no underwing tubs, better stability at high altitude)
The K-4s they received had the 30mm Mk108 or Mk103 firing through hub plus two 20mm in the underwing tubs 'greatly reucing maneuverability'
"above 28,000 the K-4 'began to float' causing the pilots to start 'swimming' or 'float' giving similar control feels to a stall"
"Un naturally sensitive" in formation flying at those altitudes.
Interestingly enough on page 313 he notes that II./JG26 pilots reaction to the new Fw90D-9 was also mixed. Loved the speed, climb and acceleration but deplored the reduced roll and turn capability from the Fw190A's as well as less firepower..
In addition, they were concerned about engine reliability and noted that the first mission of I. Gruppe in new D-9s was a disaster, 8 aborted immediately with engine problems and 4 went down with engine failures out of the 18 that took off on 24 December, 1944.
All pointing out early gestation issues in the up-engined/modified 109s and 190's - just like the 51B's in Jan-April, 1944.
Anyway - all anecdotal. My father's own experiences with the later model 109 (I think the two seater at Gablingen was a modified K) was that it was Extremely Stiff in high speed, High G maneuvers above 20,000 feet. I don't recall ever seeing any mod which aleviated this issue in air to air combat with the 51.. but it would outclimb a 51 in a tight spiral right.
At the end of the day it would be a pilot skill issue on a level playing field. Neither ship (51D/K vs 109G-10 or K-4) really had a compelling over all edge... probably true for the 190D-9 vs the 51 also.