These hardcovers collect approximately 15 issues right?
So Vol. 7 will likely collect the main-ongoing stories Order from Chaos (5 issues), Leatherhead (3), Fox Hunt (2), Chasing Phantoms (4) and the Christmas Special
Then I'd guess Vol. 8 would have TMNT:B&RDE (5), TMNTU The War to Come (4), TMNTU Leatherhead & Old Hob (1), and then maybe the Desperate Measures arc (4) from the main series and the TMNTU Wyrm issue? Not sure how'd they'd want to divide up the issues that came out around then
(I'm looking at the
IDW Chronology to see the likely divisions)