It won't be an issue on either. If I recall correctly, your 3600s don't have PVA either - double check against the platform guide to be sure. Fast L4 simply puts the LTM into delayed-binding mode as opposed to full proxy mode. PVA offload or not is sort of an academic distinction for this particular use case, and offload per se doesn't apply to either of these systems or the use problem at hand.
Also, it sounds like you may have changed the timeout value on the global fastl4 profile. If you did, I'd roll that change back, create a custom profile based off of fastl4, and customize your timeouts there. That way you can be extremely granular with regard to which timeouts are applied to which VIPs.
Good luck!
-Matt