You normally wouldn't fly this way, but sometimes it's necessary - a couple examples:
Climbout at full throttle, can be tac or fighting wing - the wingman will burn close to the same amount of fuel as the lead, and should be able to stay in position using pitch (need to speed up, gently nose down ... need to slow down, gently pitch up). All flying must be gentle. Same in case of a very high-speed intercept.
Combat in fighting wing can be the same deal, here you have to use angles to stay in as well, and it's more difficult, but not impossible ... again, fuel born can be similar depending on how both planes are flown.
I would assume that both aircraft are similarly loaded, if they're not, then the results can be predictable as you said.
This statement puzzles me a little because the ability to stay with your lead if he is at full throttle is at least partially dependent on the performance capability of your aircraft (ie - his plane has less draggy stores or more hp/thrust). I've always read and heard that flying wing on a someone using ultra high power settings results in significantly more fuel burn for the wingman.