KV-5′s pros over IS-6:
230 more health points
5km/h more top speed (better downhill speed)
0.06/0.1/0.56 better terrain resistance
nearly double total weight (much stronger ramming)
much thicker armor all around (can bounce even the BL-10 without angling?)
2mm more gold ammo pen
250 more damage points per minute (gets even higher with reload time reducing equipment, etc)
0.02m less min aim circle (accuracy)
0.54sec faster aim time
good gun stabilization (can fire on the move with precision)
double ammo capacity
very cheap standard ammo
2 more crew members (faster crew training)

IS-6′s pros over KV-5:
1.61 more power/weight (better uphill speed)
3km/h more reverse speed
2.75 d/s more turret traverse speed
better camo
8mm more standard ammo pen
90 more damage points per shot
stronger HE

so -
KV-5′s better terrain resistance lets it move nearly as fast as IS-6, and possibly faster when on soft terrain.
IS-6′s lack of weak-spot gives it an edge, but only against low pen guns (200+ pen guns can easily go through unless hit at an angle).
IS-6′s all-out worse gun handling + less damage/minute makes it very hard to do damage even with the damage per shot.
KV-5′s box shape lets it angle to increase armor effectiveness while angling with IS-6 doesn’t help much.
KV-5′s very good rear armor allows it to hide its biggest weakspot.

Are they talking about the new game mode for clans? possibly to be called clan battles?

The E100′s upper plate is thicker only because of its slope, the E3′s is as thick only in a very small strip on the “beak”, but granted it is as thick. Both have big ol’ easy-to-shoot lower plates though, while the T95 doesn’t, so…?

If you bring HEAT into the scene, the E3 can also be penned straight through its mantle, and the E100 through its turret face, so who really cares about their upper plates when there’re far more accessible parts to shoot? The T95 gets into tier 9 games where’s it’s less likely to deal with scrubs throwing loads of pure HEAT around; its armor is stronger by virtue of dealing with less powerful guns half the time.

Above all, every other tank destroyer you brought up can be penned fairly easily by the T95 (provided it doesn’t shoot their rather obvious strongest armor sections), while – as long as they miss its cupolas and aren’t below to shoot its tiny lower plate – it can pretty much ignore them and most other things firing AP and tier X medium APCR. The FV215b 183 is about the only thing that really threatens it with credit ammo, and even then it has to hit the small area around the mantle with its derpy 0.4 accuracy.

THAT is why it’s so slow and unmaneuverable. A fast T95 would be overpowered, especially at tier 9.