Overclocking memory reduces performance?

[Goose]

I've had my HIS 280x for a couple of days now and I have the core running at 1200mhz which im perfectly happy with. However, if I try overclocking the memory from 1500 to 1525 i actually get a Heaven score about 15 points lower which doesn't make a great deal of sense to me.

Can anyone explain why this might be?

 

[Goose]

I didn't notice any artifacts or stuttering and i'd be pretty disappointed if I cant OC the memory at all, especially as its meant to be Hynix.

Here are my results so far;

1200/1500 - 1130
1200/1525 - 1117
1200/1550 - 1122

+25 over stock is worse, +50 over stock is less worse but still not as good as stock :/

 

[Goose]

Update:

After running Heaven my VRM max temps were 74 and 64 it doesnt look as though overheating is the issue.