What tools you can recomend for firewall and/or VPN perfomance testing (TCP & UDP traffic) ? iperf?
I know this doesn't answer the question you asked as such, but I always recommend to clients that they use their normal patterns of load to test the performance of their VPNs / firewalls / etc.
For all these things, and VPNs in particular, they will have been installed to meet some business need. If it's to support sales roadwarriors accessing a sales app via a citrix server, you need tools that test that usage case; if it's to support remote developers accessing local source repositories, sshing into dev boxes, running compiles, looking at huge logfiles, etc., you need tools that test that usage case.
Yes, this more difficult to do than to point netcat at a VPN and squirt traffic down it, but it's also not very helpful to say "we certified the VPN to support sustained UDP transport at 1.5Mb/s" when the users are saying "ICA latency is very high, the mouse lag on SalesWarrior is unacceptable".
Test with normal usage traffic (and preferably, more of it than you expect, so you have some idea how it will degrade with heavy load).