[GuidoK]

This is old news.
If it was a viable option, it would be here already. Looking at how long it took BMW to decide for a manual option on the z4 m40i (and sadly only in the US apparantly), a decision like this would take an eternity.
And if they'd make a very limited production run, you wouldn't be interested as nobody would be able to afford it.
The estimated 150k is very very low imho, looking at what the limited BMW 3.0 CSL costs (50 made, price around 800k I believe?).
So my price estimation, if a z4 like this would be made, would be around 500k. And it's not special enough for that.

A model like this is only interesting if you're gonna make thousands, and that decision should be made way earlier in the production run, before the facelift. So too late for the g29 platform.
And sadly cars like this are currently notoriously unpopular, so it would be a moneypit. People currently want sedans or SUV's. Even the estate model, once very popular in europe, is having a hard time.