Yes it is performing fine, even for a 1 time call in.
It wouldn't affect the smaller game modes at all, where it in most cases costs you the game if you go for a second JT. The unit is, unlike the other heavies you listed, problematic in the larger game modes, and any of the buffs you suggested would probably make it a borderline I win-button.
I don't really like limitations such as these, but for the JT I would actually approve it over a flat performance nerf as it is only a problem in the larger game modes. Doing it with a performance increase would nullify that change.
Ok are you saying is over performing if so tell me how can it insta kill inf ?
The only 1 time call in is tiger ace is the JT like the tiger ace
Or cohv KT
And cost ton of fuel