1) Every man and dog knows about 3389, so don't open it on the external side.
Hackers will use the path of least resistance, unless there is something in particular that they are looking for on your network.
You can use your own meaningful scheme if you have multiple servers.
eg) 192.168.1.100 = external port 57100
192.168.1.101 = external port 57101
2) A few more risks I can think of
a) User leaves workstation faling to close session
b) Someone spies on user typing password.
- But I don't think these additional to running accross VPN.
BTW: You should notice a speed increase outside of VPN!