I can't seem to find the sheet Lone Star Merc did way back in the days of the Iron Man leagues, if I manage to dig it up I'll repost it.
But he had gone through most of the possible permutations of subs, using speed scout on breakaway plays to monitor the speed loss vs in their natural position. I do remember the most "lenient" OoP penalty started at 5%, so have to agree with Brew here.
If my memory is right, they went up in +2.5% notches, the further away Bort deemed the substitution from the base position. I also seem to remember the opposing defender slot typically, at a relatively low %. Like TE <-> LB was one of the better IronMan swaps, and CB <-> WR, you took a lot less of a hit with those, around 10% or so, where most O <-> D subs, were more like 20-25% hits semi-crippling the player ...
I often make use of FB <-> TE on things like goal line blocking, but it does seem pretty noticeable the drop off shoving a scat in at rTE vs their normal spot, and vice versa. If you're shoving a bFB in at GL TE3, and they're running outside blocker VA, you're really not going to *feel* the drop off, since they'll typically be flying around at warp speed anyways.
I've used rTEs at scat FB at times, due to a team not running a sFB on the roster. I'm not a fan, it feels like the penalty is ... not negligible to me.
I do think the spdWR SA tree is a decent upgrade over the recTE one however, just going by pretty much every elite recTE having 2 of 3, if not 3 of 3 of (Juke, Catch Fake, First Step) added, and WRs not having to dip that heavily to make a cohesive build. But not enough for me, to outweigh the OoP.
YMMV, good discussion though.