In reply to geordiepie: Great question, and one that I've been pondering myself for some time now.
I used to know a guy who worked in Alien Rock 2 in Edinburgh who swore that climbing higher volume, below your grade yielded greater improvement than projecting, or climbing less but harder.
I've tried both, and I think the best approach (at least for me) is a balance between the two.
I now try to have targeted projecting sessions where I'll try V7-V9 problems over a number of sessions - this is proving quite productive, and I'm now close to linking 2 problems after 4 sessions in total (2, ~1hour sessions on each).
I intersperse these sessions with much easier sessions, where I'll not attempt anything above V4, but I get a lot more done. These sessions improve endurance, and give me a bit of a boost in satisfaction by topping out something!
Looking forward to seeing what everyone else thinks about this!