RE: 49 neptune value hobby motor to weight?
It seems a little heavy to me, but the basic seamaster design is very forgiving on weight. The tip floats help trap air under the wing. If you look at the Wigdon, the new GP twin, it is listed as 64-72 oz for only 375sq inches of wing. The neptune has a slightly shorter wing, but it's quite a bit wider, and at least claims to have 500sq inches of wing to the wigdon's 375.
Easiest places to save weight are in batteries, motor and servos. You didn't mention what you are using now, but anywhere you are using full sized gear, you can reduce and save weight.
If the plane is ready to fly, and you know how to fly, I would just throw it in the water and try it. You can get used to taxi, getting up on plane, doing big power slides on that flat bottom. Be sure your battery is secure and the mount is strong so it dosn't go through the hull if you bounce hard.
If you find yourself in the air, just keep your speed up untill you have a chance to get high enough to try some low power stalls, that will give you an idea where your stall speed is. Alternitivly, you can just come in for a landing, hold at about 12" off the water and just let it slow down, adding elevator untill it plops down. At that hight, even if it stalls, it will just land. The neptune will not catch a tip float, so it's a pretty safe plane to just fool around with.
Hey, just noticed you are in Wa, if you are near me, you could bring it over and fly it off the small lake I live on.