I have heard this all from people who work at GWR. 175s and 158s reportedly denied due to being non-standard (158s because Perkins engines and 175s for obvious reasons) and GWR want 150s for a quick entry into service as they need stock ASAP
158s engines aren't non-standard, they're the same as the 165/166 fleet. That's most likely train crew rumour mill rather than anything substantial.
The most likely situation as regards 158s is that the TfW fleet will go to EMR (due to ERTMS fitment for operation on ECML). Anyway, I don't want to get into the world of speculation!