As I understand it, the MTP fabric isn't rated the same for fire protection as the RN PCS fabric - it's also why it's worn with a cotton t-shirt instead of the wicking one worn with MTP. Horses for courses.
I don't see any problem with tweaking the current PCS, but as per
@jrwlynch above it shouldn't be a separate rig, just a gradual replacement of existing stocks with either considered correct for wear (indeed, there are certainly at least two versions of MTP PCS, both of which are fine for wear).