The 13 as stated by others is much faster (just better all around) than the 12m of last year or 06.
I think a lot of brands had a big step up in 08 over last year. Best taking the largest step up, imo, which is why you are seeing guys that have been on 'roos for a while honestly ecstatic over this year's kite.

My personal take on the upper ends of the 08 vs. prior years of the Waroos --

In the past, you could just sheet out the Waroos ad-naseum and spill almost 100% of the wind off the kite. It was semi-ridiculous. This allowed a rider to depower the kite a lot and have an over-sized kite out in some pretty serious wind. When fully depowered to this point though, steering was sluggish (duh... lol... no rear line pressure) and the canopy distorted quite a bit with flutter and all sorts of stuff. I personally always took myself off the water on a Waroo in these situations long before I felt overpowered. I would just look at the kite and sense I needed to not be on a kite that in my mind had way more wind rushing over top it, than underneath it. You could literally see the top of the canopy becoming depressed and deforming from all the wind rushing over top.
I have seen too many bad things happen in that situation with kites.

I saw Tom ride his 12m in 30 to 35 knots once. It picked up on us from about 20. It was pretty sick. He ultimately came in and I grabbed his kite. I was LITT on a 7m Waroo at the time!!

The new 08s won't let you do this (go to silly top ends). The kite is forced to stay in a position where it is more stable this year (ie, won't deform and do the "whacky"). So, you will have more wind rushing under the kite proportionately than in the past when totally sheeted out to the point of slack rear lines. This means you are going to get blown off of the water when "you should" get blown off of the water on the '08 kites. I personally think this is a good thing.

As Tom said, depending on your size etc. expect to feel uncomfortable with the 13m's top end long before the 12m kite's top end.
Also as he said, there is plenty of overlap b/n the 13 and 9.

I hope that made sense. I brought it up because the top end is going to feel different to you, I think.

I think Best did a very good job with their wind range chart this year. SQ posted them here -- http://www.kiteforum.com/viewtopic.php?f=1&t=2345359