But when I use a PPP-Profile associated to this L2TP with default-encryption set to yes, I guess it is encryted and somewhat secure? Would that default-encryption setting benefits from the hardware assisted chip in the RB1000?
Yes, but that PPP security is technically outside of L2TP. L2TP is often used to transport PPP, which can offer some encryption (though by comparison to other protocols the encryption is not particularly strong, but it may well be sufficient).
I don't know if that can be hardware offloaded on the RB1000.