i have toyed with a 3 man at the 4, i think it is generally hard to pull off because of the rebounding hit, but, a couple of the best all time power forwards ive had were small forwards. my ideal team would have a 3 at the 4, i just don't usually have the luxury of doing so. i actually find sfs more useful for the 4 than the 3.
i don't follow you on the why it is good though... the part about added ath and reb. i see the ath/spd, but reb? isn't that the primary reason not to?
i'm glad you brought up the per in your bigs, i've heard you say it a dozen times and it is definitely a difference between our teams. i virtually ignore per in my big men. also ball handling. however, i do value passing. i like your idea, about any guy who gets the steal being able to take over... also maybe because your big men can shoot, it allows you to get a better look more quickly than me (because you have more options). on the other hand, because i have no bh/per, and generally care about passing, i think it is one more reason a team like mine might make better use of the extra time looking for a shot.
one final point along those lines is, i greatly value reb and def in my bigs over offense. it is because my team reb is weak, and if you can't rebound, it doesn't matter if you can do anything else. so once i focus on the reb, i am generally without great offensive bigs (d2 more than d1, but some of both), so i take good defense at the expense of offense, figuring they are already disadvantaged enough that i may as well let my guards take the shots. so, maybe part of the problem is i have fewer options - again, making the extra time more important to us.
i wonder how many with a lot of uptempo success tend towards balanced vs imbalanced offenses? i play a healthy mix of both, but since potential, i have leaned more towards imbalanced. meaning i only have 2-3 scorers on the floor often instead of 4-5. a couple of the best teams i've ever had were d1 balanced motion teams, but i never gave uptempo a serious try, i regret that now.