Agreed you won't need much to max out a 100 Mbps connection.
I have a few relays in the ~ 200 Mbps ballpark running on single core VPS instances. The underlying hardware is based on Intel Xeon E5-2650L.
CPUs with AES-NI help. 2 cores may offer marginal improvement. More than 2 cores isn't going to help at all. Tor still isn't optimized for it.