OK, first on the WEP duration/recharge issue. As it has been correctly pointed out, the early DB605 did not use boost additives. So that truly cannot be the reason. Secondly, being licence build, is not convincing also, since the C202, 109E and Ki61 have the same WEP parameters, but 2 of them have licence build 601s.
Now on the speed issue. Well at least in AH, the amount of drag on the 109G2 and the 205 are about the same, with a slight advantage for the 205.
Thus, the 205 on MIL settings is indeed a bit faster than the G2. On WEP this is completely reversed. Why is that?
Well, you could say that altough the 205 makes same power on MIL as the G2, it makes less on WEP. However, the E6B shows both engines run on pretty much the same settings, and therefore the same power. So the "less HP" theory is rather unlikely.
On the other hand, when both planes are loaded to the same weight ans thus equalizing the the P/W ratios and also the wingloading, the 205 easily outclimbs the G2, indicating it is actually making more power, which on the other hand is completely lost in the speed performance.
It is really strange and I am with Anaxogoras on this, the C2 do really need an FM review, not just a 3D facelift.