While the S85 V10 is a big engine, I'm not sure how much it weighs over the S14 that originally came in that E30 M3, which I'm pretty sure was a cast iron block. The V10 is cast aluminum; if they mounted the motor as far back and as low as they could, handling could still be pretty neutral, I think. That would largely depend on what kind of weight distribution they were able to achieve. Also, we're dealing with weight up front with (massive) power being driven from the rear. It's not like a big heavy motor going into the nose of a FWD (where big understeer would be a problem).
Since they changed all the suspension pick ups and swapped subframes with E90 M3 parts, it would be interesting to see how this thing behaves on the track.