I don't believe he will need to rejet, adjust his idle mixture screws to about 2 1/2 turns out yes but not rejet unless I am missing something.Since you have changed the dynamics of the exhaust, you will need to rejet the carbs. That much I do know. What size jets is for someone else to answer.
You could very well possibly be correct. (Anyone that 100% sure can pipe in anytime) I am mearly going by everything that I have read about installing aftermarket pipes and what is suggested to be done when doing so. Ear shaving I am aware in that proccedure required rejetting but pipes required adjustments of the air mixture screws.... One question asked and now I have one myself... Damn you MAN, thanks JOEY...lolUnless I am mistaken, which is a possibility, this is just about the same as degoating which requires a rejet AFAIK. In taking the mufflers out of the equation, you've taken away the back pressure the mufflers create. This changes the air fuel mix and not just at idle which the only thing the air mix screws adjust. It changes it at all RPMs, and therefore he may need to rejet.