150 is pretty high for anything more than short bursts! Do your spin bikes have cadence meters? Or are you estimating? The spin bikes I have used have never had them.

When I ride my trainer at home with Spinervals I watch my cadence meter and heart rate meter. One drill I like is 30 seconds on / 30 seconds off. The 30 seconds on is broken down in 3 segments of 10 seconds. First 10 seconds at 80%. Second 10 seconds at 90%. And last 10 seconds ALL OUT 100%. Then rest for 30 seconds. When you know you only have to REALLY crank it for 10 seconds you see how fast you can superspin!