Do a search. I've posted my opinions in the past.
To make it short, don't put non-M Z4 H&R springs on your Z4M.
- They will slam your car. The 1.5", 1.25" is more like 2" & 1.7". Your stock struts/shocks won't last more than a few months.
- They're capable of being installed on Z4M struts/shocks, but the spring rates match up the stock strut/shocks poorly.
If you just want some aftermarket springs, the H&R springs meant for the Z4M are actually pretty decent. Many forum members have those on their Z4M's (do a search in the forum). As far as I know, only 1 person has complained about those H&R springs, but he got one of the first batch and the springs were faulty.
Or you can go the coilover route - TCKline, KW V3. But don't bother with the H&R coilovers (POS and just about the worst coilovers you can put on a BMW).