The mobo will select the easiest choice from the SPD if you leave everything on AUTO. If you bought 1600 and WANT 1600 performance, then it's quite likely you will have to set everything MANUALLY.
Performance difference depends on YOUR NEEDS. Don't let us make up your mind for you. If you know you will need the extra bandwidth, do it. If you're just mucking around for the usual office/web surfing junk, then personally, I wouldn't bother with the hassle.
1. INTEL i7-920 C0 (200x19 @1.275V), Gigabyte EX58-UD4P (F14p bios), Corsair Ballistix Tactical Low Profile 2x 8GB DDR3-1600 (7-7-7-21-1T @1.35V), EVGA GTX Titan SSC (1205 MHz @ 1.212V)
2. INTEL X5650 (182X22 @1.336V), Asus Sabertooth X58 (1402 bios), G.Skill RipjawsX 2x 4GB DDR3-1600 (9-9-9-24-2T @ 1.65V), Sapphire Trix OC 290X