I've got a Seagull Extra 300. I am also putting in a Saito 91. I had a Grauper Extra 300 which is the exact same plane as the Seagull, just slightly different color scheme. The build is exactly the same as well. On the Graupner, I ran a Saito 90 twin cylinder producing about 1.2HP. The plane flew well. Vertical was good, but a little more power would have been nice. Due to the motor being a twin, an onboard glow driver system was installed and that added a little weight. The plane AUW was 7 pounds 14oz dry with that motor, the onboard glow/extra battery and an oversized 16oz tank(compared ot the stock fuel tank).
The Seagull I am doing now will not have onboard glow since Im using the single cylinder 91. This motor produces 1.6HP so the vertical will be a lot better, the plane will be almost a pound lighter and so overall performance would be very, very good. Even so, with the less powerful twin, the other plane flew very well and was responsive. It was easily a good pattern plane. With the lighter weight saito 91, you should expect about a 6.5 pound plane.
Here is an online review of the Graupner Extra 300. Again, this is the exact same plane as the Seagull(different color scheme) so it translates 100%.
http://www.rcuniverse.com/magazine/a...?article_id=61

here is another review, except this one was converted to electric. but, you'll see the build quality in the pictures.
http://www.rcuniverse.com/magazine/a...article_id=364

I opted to change the spinners on both planes to aluminum. The graupner has a truturn and the Seagull has a Great Planes. Both are 2 3/4".