George I don't think you will notice much difference in how the plane flys without the cowl on, I'm not sure why I think that but after you fly it you will see what I mean. I ran 425's for ele. and and rudder and 1 605 for the ailerons, I take it you are going to mod the ailerons and run two servo's, good idea when I put mine back in the air I plan on doing just that. It has been a while since I assembled mine and can't remember how much weight I had to add with the Webra but to me any extra weight is too much and that's why I went with a 61 on it. As far as the airframe being able to handle your 90 there is no doubt in my mind that it WILL handle it, the Ultimate is one of the best built arf's that I have put together that much I do remember. Make sure your battery and reciever are strapped down tightly, the ultimate will do some of the most violent snaps and spins I have ever seen and that's why I have not flown mine for a while, during a beautiful blender something came loose inside and I didn't have total control after the spin and I set it down in the top of a tree, it ripped the mount off on the bottom wing and loosened the cabanes in the top wing, very tough plane. I am thinking of a new wing kit because I'm not sure if the wing will hold up after any repair to the mount section. Yes I did have 2 degrees of upthrust on the Webra, that is automatic with me when I build any high performance plane and I can change it in a few minutes if it doesn't seem like the plane needs it, vertical was not real impressive with the Webra because of the extra weight needed for balance, different story with the 61. I didn't glass or pin the firewall I just added some corner round and a little extra glue and it was fine. Good luck, hope this helps and keep us informed on how it goes.