According to what was said by (GBRf's) Bob Tiller in the recent
Green Signals video (from 4:49), raising the top speed only involves changing the bogie dampers and a software change. So the 99/4s should be just as capable hauling freight as the 99/0s when they're not required for passenger use, and will doubtless do so. In reality, at 113t the 99's are lighter than or similar weight to a class 47 or 57, so should be fine passenger locos. I'm a bit surprised the data table on the cab side (shown in the video) says RA7, given the sub-19t axle-load (which is nominally RA5/RA6 territory):
View attachment 183217
Also, the current GBRf sleeper haulage contract announcement in 2023 (for 7 years) mentions the possible use of 99s in the future -
https://www.gbrailfreight.com/gb-ra...al-to-support-new-era-for-caledonian-sleeper/ , so the 99/4s should be hauling more than the Belmond trains in the future.