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The Fortigates should do as good as any.
Can you get one in on good RMA terms and try it?
Or do you have a spare boxe(s) around to give OpenVPN a spin? To test slowdown anyway...

How many concurrent users talking over links as an average?
What applications/protocols will be using links?
VPN overhead might chew up more avail bandwidth than you want.
Do you have budget? Do...