I haven't set up a MS system myself yet, but I do know low impedance will work just fine. My personal preference is low impedance for the reasons you list and quite simply because in the circles I work within they're easier to come by. The resistor pack is not a big deal at all and isn't anything which would affect my choice.
You're not going to need any larger than stock GTE injectors either... Could buy a set with a couple extra get them all serviced and blue printed and have a set of matched injectors for a lot less than new if you're looking for reliable but inexpensive.
I hope you're at least using NEW and moly coated pistons if you're using cast, GE spec pistons.... I've see a large number of pelted 7M-GE pistons all based on the whole "I'll only be running Xpsi". I happen to have one sitting on my desk which was ONLY running 8psi on a very well tuned engine. Water/methanol injection will help a little, but it has it's downsides too...
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