I've had a couple of Phoenix planes- built very well, a tad heavy though the Extra is only 3.5lbs on Towers site (misprint?). If I were to get the Extra, I would use a 61. Never hurts to check the firewall and add some more glue.

However, something sounds wrong with your 46AX not getting planes off the ground. I fly off a grass strip and my 46AX pulled my Nexstar, Dolphin, and Venus 40 off with no problems - 3/4 throttle for the Nexstar, less for the Dolphin, and I use full throttle on the Venus just because I'm AMAZED by the ground speed I get in such a short distance!

What prop are you using? The Venus 40 specs heavier than the Somethin' Extra, so something is definitely wrong with your setup. I broke the 46AX in with an 11x5, and switched to a 12x5 after 5 or so gallons (10% nitro) - plenty of pull for a 40 size plane.