Depends which cost function you take into consideration. The size of the block part occupied by a multi-transaction would still be O(n) as are n transaction. So, yes, it could be cheaper by a factor of 2 or so.
A key thing that Nxt *has* over its rivals is fast txs. If I have a tx with 100 outputs and 1 sig vs. 100 outputs and 100 sigs then the latter is going to be slower both in terms of data size and sig verification.
So I think that Nxt should allow such more compact txs in order to provide the "leanest and meanest" tx system in the crypto currency world.
Agreed. We might as well be the best in all areas.