There is no way to have different throttle the bandwidth used by clients to download policy updates and log submission from clients to SEPM per group. Anyway they are very small in size. The definition updates files are the ones that are sometimes of considerable size.
If you desperately want to throttle the bandwidth, used for DEFINITION download, per group then consider making other servers (or even PC) that are in the same site as the SEPM in to GUPs. Then you can assign one GUP per group and throttle the bandwidth used by each GUP per group (for definition download) in the the GUP policy.
This will help to manage the bandwidth utilized via WLAN links.
Note: In the above case, you cannot have the GUP server with different throttle settings in the same group.