My opinions from here on. Read into them what you will
When I started flying IMAC, I had a Goldberg Ultimate with a Saito 150 and standard type servos. That plane was a blast to fly and had TONS of power. However, it was difficult to fly through the Basic sequence and even more difficult to fly through Sportsman. I have since found out that with good servos, it would have been much better.
My second year I flew an Aeroworks 29% Edge in Sportsman with a BME 50 and good, digital servos. Wow, what a difference. The plane flew SO MUCH more smoothly and drew much better lines. I found myself not having to work as much to make things smooth.
Last year I flew a RadioCraft 35% Extra in Advanced with a DA-100 and the best servos I could buy. Again, a quantum leap in flying qualities. Slower, smoother, much more graceful and stable.
This year I have just finished a 40% Extra that I plan to fly in Advanced and then Unlimited. I have heard that I will see another quantum leap in performance. Can't wait to find out.
Having said all that, you will definitely notice a difference as the plane gets larger--assuming that you can outfit it with the appropriate servos. Since this message is in the IMAC forum, I am assuming that you will be flying precision aerobatics (or at least you are thinking about it!). If so, DON'T SKIMP ON THE SERVOS. You can't believe how much of a difference a good, coreless servo makes. Hitec makes some really good quality digitals that lots of guys are having good luck with. Go ahead and buy the 59xx series as these center very well and have great resolution and power. You will find yourself fighting the plane alot less. I don't have personal experience with them as I have up-to-now stuck with the trusty JR8411's.
It sounds like you don't mind building if you are considering the Midwest kits, but let me make a suggestion. I would seriously consider the Aeroworks 29% Edge with either the BME50 or the Taurus 3.2. This combination seems to fly much "bigger" than the Midwest offerings. You will only need 1 servo on each aileron and 1 on each elevator. You can easily make do with 1 high torque (Hitec 5945 or JR8411) on the rudder. I used a 1650 mAh NiMH pack (5 cells) on my Edge and I could easily get 5 flights on it. This pack should run you about $25. Put another one on the ignition. This plane/engine combination is great for IMAC sequence or for 3D. Gas will run you about $2 per gallon (after you add the small amount of oil) and you will be getting about 1 oz./min of flight time. So 5 gallons may last you all summer depending upon how much you fly.
You asked about the price jump, and honestly this depends upon the hardware/servo choices you make. The initial cost may be higher, but if you fly alot, you will save tons of $$ in fuel. And if the plane is well maintained, there is a good market for them when you choose to sell it.
Above all, have fun and don't be afraid to ask questions.
Ken