Somewhere in his book, Ron Daws mentioned the difference between 20X400 with 100 recovery vs. 20X400 with 400 recovery. They are basically completely different workouts (duh!). I can't really speak in terms of energy system but, naturally you're not gonna be running as fast if you're having only 100m jog for recovery. I think it all depends on what you're trying to achieve with that particular workouts, how would your body react to it, and where you are in terms of developmental stage. Daws said something about 20X400 with 100 would be a good workout for during the conditioning period but 20X400 with 400 jog would be a good workout for anerobic phase. I personally don't think it's a good idea to shorten the recovery in order to "slow" down the pace. My personal opinion...