WAEs are not restricted in terms of hardware and software WAN / LAN Bandwidth. There are recommendations that should be followed when using the full suite of optimizations.
For instance the 512 will utilize up to 8M with 1G of memory (20G with 2M) of bandwidth for DRE and PLZ.
In addition to bandwidth, each WAE will need to accommadate the number of required TCP seesions, 600 (1G) and 1200 (2G). If the max is reached, new sessions will enter passthrough until optimized sessions is reduced.
Attached is the sizing guide.
In terms of TFO optimization only (no dre, plz) the BW numbers increase as follows:
WAE-512-1GB - 350Mbps
WAE-512-2GB - 400Mbps
WAE-612-2GB - 450Mbps
WAE-612-4GB - 500Mbps
One other thing to consider is the amount of current BW utilization on the links. If only utilizing 20-30% of of the 40M the 8M optimization is fine.