It all depends on where you start. I think it's possible to improve as much as you are willing to work for. As for me, My original diagnostic was a 146. I screwed around with my prep and got a 152 on the Dec. 06 test. Big mistake. After that, I made the choice to buckle down and put in the work and it has paid great dividends. Before the Sept. test I was preping at 168-169 consistantly. I had a bad day (-8 on games, normally - 1) and got a 163 so I'm retaking. The great thing about it is that I have the chance to get even better before December. It's an opportunity.
I'm not that smart. I don't think I'm diferent from anyone else. Definitely not trying to talk myself up. Their are a lot of people doing better than me. The point is that it's possible. If there were ever a time to insert the "if I can do it anyone can" cliche this is it.
Does it work for everyone? Of course not. There are countless stories of people who beat their heads against the wall for months and never improve. Who knows maybe I screw up another section in Dec. and don't help myself at all. Nothing is guaranteed. It is however resonable to believe the only limits you have are the ones that you place on yourself. Need a 25 point increase? Go for it but be willing to put in the time.
I will say that those last few points are the toughest to get. After countless 169's I would probably give my first born for a one point increase. A 178 would probably work for me.