Those who buy a HP Microserver Gen8 usually go for the entry-level model with Celeron G1610T processor. Since the CPU is socketed, it can be easily replaced.
After some research, among other things in the very good collective thread of the Hardwareluxx forum my choice fell on a Core i5 3470T: 2 cores, 2.9 GHz base clock and hyperthreading support (4 threads). The TDP is also at 35 watts, so that the same cooling system can be used.
With FreeNAS 10, the things for which I currently use a Raspberry Pi: an OpenVPN server, DNS caching, Bittorrent sync, backup etc. will also be available. For this the performance is then certainly useful.