The Tempest is a very good fighter, but so are 109K's, 190D's, and Ta 152's. Also, Tempests are not the best above about 20k.
For example:
and
In past scenarios, Tempests are often flown by some very good pilots, not average pilots, so that helps the K:D of Tempests. Still, there are other groups that do well also.
For example:
-- In The Final Battle, one group of Tempests got an overall K:D of 2.4 and another got 3.8 (which is great). However, I/JG53 got 1.8 in its 109K's; and Stab/JG301 got 3.7 in its Ta 152's.
-- In Winter Sky, Tempests got a K/D of 1.2. I/JG27 in its 109K's got 1.5. I/JG26 in its 190D's got 2.0. III/JG11 got 2.0 in its 190A-8's.
Tempests are good, but not overpowering in scenarios, especially if the other side has 109K's, 190D's, and Ta 152's.