The Spirit Realm

I would change the BP in a second, since at your level you should be having any real mana problems and just put a don mana pres aug in even if its only 15% that is not going to kill you, gratz on the new BP very cool.

my advice would be to equip the new BP and get the Shawl of the Void from Cyno. Its a shoulder items with MP.

DoN mana pres aug is 10%, not 15%. It's a significant step down. Not out of the question for such a high quality item upgrade, but a definately a heavy tradeoff to consider.

I believe mana preservation, like a lot of other focii, is variable. Conservation of Xegony is 14% max for level 70 spells, which means it is only 7% average. The other focus is 10% max, or 5% average. The 2% extra savings per cast on average wouldn't be as valuable to me as a really good item upgrade, especially if you are in need of the spell haste on the new BP.

7.5% average, actually, since the minimum is 1% savings, not 0%. Compare that difference to Secondary Forte AA. 9aa points for 2.5% mana pres on one type of spells. 2.5% (average) on all spells is even better.

Fixed numbers; true, but then MP5 averages 5.5 rather than 5, so it's still 2%.