With my RAM to 3800 and IF at 1900 (3800X) I took a performance hit compared to 3733/1866. In Cinebench anyway (~300 points). I also needed 1.49V on DRAM to sustain 3800MHz. Since 3733 is faster and the performance difference between 3733 and 3600 is naff all I've stuck with 3600/1800.
After some further testing, ive gone back to 3600/1800 as was getting some odd issues with my Soundblaster card at 3800/1900.