Well here is my take. The idea is a good and sound one. The imp case is tailor made for the handi platform. Considering the alternative in factory form is the 308. While an excellent cartridge in its own right, its rimless. Yea yea, I know many many people use rimless handis with no problems. BUT, many others have had problems. few will argue, all these problems GO AWAY if you stay with a rimmed case. IMHO headspacing on the RIM in a handi or any brake action is THE way to go.
SO, the 30-30 AI is a good choice as would be the 30-40 Krag. Its a HAND LOADING only proposition in the Imp case, so why fret about a couple hundred FPS loss shooting factory fodder? Your making it to IMPROVE on fact ballistics, right? The ability to shoot factory stuff is good and can be done to make cases or in the event your handloads are unavailable. I say just do it the ballistics's advantage is a good one and its a worthwhile upgrade and improvement.
As for the "questions" about the 357 Maxi reaming. I completely fail to see any legitimacy to the arguments posted.
1)
Loss of ability to shoot 38/357mags with any accuracy. NOT so, many of us actually have sen IMPROVEMENT in said accuracy!!
2)
Issues finding cases. Where have you been? Cases are easily available at any one of a number of sponsors right here on GBO!!!
3)
Not a big enough gain in performance. WHAT, again, where have you been? Show me ANY 357mag that will top 2000fps with a 180 gr bullet within safe pressures. (There are none!!) THEN we will go shoot my MAXI and we will TOP 2200fps WITHIN SAFE pressures from my 21" barrel!!!
4)
Why not just go 35 Rem. NUMERIOUS reasons... First off, lets be clear. Its a GREAT old caliber and I love it. BUT, it is loaded to much lower pressures and has been PLAGUED with head space issues. Remember its another rimless case. IMHO, making it a poor option for the Handi. YEA, I know its recently offered by the factory. But time will prove this out, it too will be plagued with headspace & ejection problems and will not last long. (Where is the Whelen?) Besides, within published load data, its a ballistic equal to the Maxi and the maxi is very well liked and received from all who are so inclined to ream one. In case I didn't mention it before, its my favorite handi caliber.
CW