I am not sure what, but there is something wrong with the suspension geometry. Flex seems to be ok; I always preferred using all screws in the topdeck. But talking about the suspension, short arms in the rear make the car a little better, but do not solve the problem at all.
I had a hard time bringing the car to life outdoor. Who followed this thread knows what I tried the whole season. And even though the car behaves ok, the last two tenth were not to find without a conversion. Right now I am driving with ultra short camber links of 40-41mm length. This is radical, but brings this chassis to life (and got me the last tenth I was searching for). The T4'15 I am testing since three weeks, works extremely well with long camber links...
A friend of mine bought the Volker ETS Yokomo chassis build by Umino (who else) and copied the setup and measurements for the 6.2. He said it was a huge step forward with his 6.2. In direct comparison the Yokomo was still faster 1-2 tenth a lap.
So in conclusion flex feel looks to be ok, but seems to be in disharmony with the suspension geometry, forcing us dedicated corner speed addicts to change the car (a lot).