What’s the throuput of the RB133? Both wireless to wired ether port and wired ether to wired ether?
up to 120Mbit is possible if you don’t have any firewall, the conntrack is off and you use bridge. will decrease with smaller packets and firewall.
And What’s the throuput of the RB133C? Same of RB133? That 16MB more memory in RB133 over RB133C do diference in throuput ?
Beyonlo
CPU is bottleneck not ram 
WPA2+PPtP on both RB133 will get you (IIRC) 500 kByte/s (tested using FTP).
~45 mbps tested in UDP mode wireless bridge. Use search man.