actually the IR is VERY important... the higher the resistance the less voltage there is getting to your motor... so lets say you have a high IR ESC with an $80 world matched pack, then it'd be like you running on say a high voltage 5 cell pack cuz most of that voltage is getting eaten up by the ESC.
versus a low IR ESC with cheapo $40 stick pack, but that ESC is letting ALL the juice get to the motor..
.002 resistance difference is A LOT!
my personal example.... I run a quantum sport(very basic esc) with matched packs... after a race, I discharge that pack and find that 1/2 to 1/4 of juice is still left in it! and thats cuz i overgear like crazy sometimes.
recently, i bought a M3 cubic(high end esc) and ran that with the same pack, motor, and gearing. now, I dumped right after crossing the finish line.... all the battery's power is getting sent to the motor! WOW, but what a difference in punch!
now i got a quantum competition(high end esc too) and I cant wait to use it!
I suggest you save up and get the absolute best esc you can...you wont regret it.