I think that there is a bug with group move and speed boosts(Like those prevented by friendly command centers) working with units immune to speed boosts. But it may have been fixed in the upcoming version:
* Fixed some longstanding (since introduction of group move trying to handle immune-to-speed-boosting units) bugs with group move:
** Previously, for example, a speed-boostable 140-speed unit group-moving with a non-speed-boostable 152-speed unit would result in the boostable ship going 152, and the non-boostable ship going 140 (_below_ its normal speed!). This was because the non-boostable one was taking the minimum speed of it's boostable friends, and then setting its speed limit as if those friends were not actually boostable.
*** This bug has been captured, stuffed, and placed in the museum. It couldn't get away fast enough.
** Thanks to HellishFiend and Spikey00 (among others) for reporting.