Personally ive had mixed results with my Xigamatek S1283. I have the NZXT Tempest which is preety much the same thing as the Antec 900 except the Tempest is a little larger, and has two 140mm fans up top. Using the Xiggy and the AC7 at 3.0ghz on my Q6600 netted the same temps idle and max load, but the AC7 could not handle this over 3.0 on full load without flying past 70c in Prime. However at 3.6ghz this Xiggy keeps this at around 68c-73c in a midly hot room after a run of Prime95.
However your temps are very good, alot better than mine even after lapping the CPU and Xiggy. As long as you are under the thermal limit and hit your OC marks I see no reason to change coolers.