I don't know about the millijouls or whatever that is but I can switch back and forth between a 6AL and a 7AL box all day long and run the exact same ET. Done it several times clean down to 7 second ET's normally aspirated.

Boosted or Nitrous setups would be a different story.

67 Barracuda street/bracket car 11.27-119
68 Dart 502 BB 8.70s-152
414 cid SB Dragster 7.65-174