hudr,
Linksys tech support is wrong. I've setup a stable tunnel between two BEFSX41s (both on dynamic IP) and from an RV082 (also on dynamic IP) to each of the BEFSX41s. This is standard Gateway to Gateway configuration. There are a couple finer points, though.
1) Use DDNS setup in each dynamic IP router to keep a dyndns.org Fully Qualified Domain Name (FQDN) up-to-date with the current dynamic IP address of the router. (Setup of several such FQDNs at dyndns.org is free.)
2) Firmware for the BEFSX41: Version 1.45.7
This was a Linksys beta release, but continues to be the most stable for the BEFSX41. Links to this firmware are available at dslreports.com in the Linksys forum. Search for BEFSX41 and firmware and you'll find it. NOTE: You DO NOT want firmware newer than 1.45.7! (I caution since 1.50.18 and 1.51.0 versions of firmware are currently available).
3) Firmware for the RV082: Version 1.1.1 or later (current version is 1.1.5)
The rest is pretty standard setup stuff. If you need help, please post again.
Hope this helps!