This is very bad news for longtime Sonicwall customers. The L2TP VPN has been one of its core features which has always worked well for Apple Mac remote access. On most "legacy" and no-so-old appliances, the L2TP connection is simply much faster for most applications(especially cifs) than the (SSL-VPN) "MobileConnect" app.
Sonicwall has done their part in making it possible to maximize the security of the L2TP/IPsec VPN protocol. Apple hasn't exactly followed. Please Apple, don't discontinue the built-in L2TP client.