Stock/OEM tunes will handle up to E18 or so without need for tuning. If you did not load an E30 tune right after you filled up, that might be why the SHO seems to be bogging.
If you DO have an E30 tune on, you could try a KAM reset by pulling the neg batt cable for 40+ minutes to force the PCM to relearn. Shouldn't normally have to do that, but can happen.