Well you have to keep in mind that they would be ridiculously overpowered if they would also fare well in battle, since they're already a fast raid unit which can raze a castle or a town-center in seconds (if massed). This is of course not always doable (depending on the location of the enemy army) and not always optimal, but they have their niche.May I suggest reducing the cost of tarkans slightly (to say 50 gold, or 50/50 gold/food?), they are still a terrible choice compare to just going knights (remember, with knights you save the 650 stone, and the tech if you plan to mass tarkans from stables). Perhaps also give them somewhat equal speed to cav archers (I think they are slower), and it could be very viable to go fc -> tarkans, even against cav archers (they still lose to knights).
Almost unrelated: With the expansion the non-elite Tarkan also has 10hp more than in the standard game, but even without that they'd have their (niche) spot.
And you also have to consider the hun strength - which is already strong - if you improve Tarkans even more. Realistically you would have to remove power from the huns elsewhere.
So in my opinion, all things considered, they're definitely in an acceptable spot right now.
if knt are massed, they can raze tcs also, but the main thing imho is, that you can upgrade them to elite, which makes them really strong, so if you would make them stronger the upgrade wouldnt be needed.