It's not a great surprise that a 35iS is as quick (or quicker) than a Z4M in that track video. For a start it's at least 5 years newer technology, on a car that had an RRP £3k higher with very similar power and a boat load more torque. Frankly the fact that a Z4M can keep it honest, is a testament to the M Division.
I personally don't think the 35iS will attract much of a following as a classic in the future but it's all conjecture really isn't it? We none of us know. My reasoning is that it has little pedigree - it didn't go racing (the E86M did), it's not an M, it doesn't have any uniqueness in its engineering either - it's not the last straight six NA M car with no driver aids or DCT for example. There is no curio about it other than the fact it was low volume and top of the E89 tree.