I think you have a good combo there for engine and reciever. Personally I used the HS225BB Mighty Mini's in mine. Although I use the HS81's in my .25 size Texan, I have some unwarranted and unsubstanciated reservations for the HS81's on a highly aerobatic plane like the Extra. I know those little guys put out some decent torque, but the gears are so small. I'm just using the better safe than sorry judgement call.
BTW, you will love the plane. It is fast and highly aerobatic with a great color scheme. I had an old OS .25 FSR ABC with a pitts muffer.
FYI
HS81MG($23.99) - Length: 29.8mm Width: 12mm Height: 29.6mm 1.17" x 0.47" x 1.16"
Transit: 0.11sec/60 deg 4.8V
Torque @ 4.8V: 2.6 kg-cm, 36.1 oz-in
Weight: 19g 0.67oz.

HS225BB ($20.99)-SPECIFICATIONS: Length- 32.4mm 1.3" Width - 16.8mm 0.7" Height- 30.8mm 1.2"
Speed - 0.14 sec/60 deg.
Torque- 3.9 kg-cm 56.4 oz-in
Weight- 27g 0.95 oz.

There is plenty of space in the Extra for Standard Servos so the only gain you are getting from the HS81MG's is 1 oz for 4 servos. at the cost of 20oz-in of torque. Plus the HS225's would be $12 cheaper.