Today I did a side-by-side film speed type test comparing SD-28 to D-76(1+1). I shot two rolls of Ilford Delta 100 rated at box speed. The relative exposure of successive frames was: 0, -3, -2, -1, 0, +1, +2, +3, +4, +5.
I processed one roll in SD-28(1+0) for 12 minutes - this is the mfgr's recommended time for D-76(1+1) and,
I processed the second roll in homebrew D-76(1+1) for 11 minutes - the mfgr's recommended time for ID-11(1+1). I used D-76(1+1) as my control because this is my normal developer.
I have not yet printed these but, looking at the negative strips, I see very, very little difference between the two films. There are differences...but they are very subtle and really very difficult to quantify. There may in fact, be a slight loss of speed with the SD-28 - but, if there is it is insignificantly small. Certainly no more than 1/2 stop. I am surprised to report that appears to be no significant difference (again with the naked eye) between the two with respect to base fog.
The main advantage of SD-28 appears therefore, to be in it's additional buffering capacity - which is irrelevant if not used in a replenished system as far as I am concerned. Prints may say otherwise however.
Oh, so in short, for SD-28 straight, I use the mfgr's recommended time for d-76 (1+1) -- as suggested by R. Suzuki.