I've had 2 births with SPD. It affected the positions that were practical (I had both kneeling down leaning against something which didn't cause probs for the pelvis). I didn't want an epidural but if you do, they recommend measuring the max comfortable distance between your knees so the midwives can ensure you don't overextend while you can't feel it. Other than this, I think there is a slight beneficial effect- the additional movement in the pelvis means there is a bit more space for baby.
I had awful, debilitating SPD throughout my second pregnancy. I'd been told to warn the midwives if I had an epidural (as above), but I managed without and gave birth on a birthing stool (highly recommend btw), to a 10lb 5oz baby without any issues.
I gave birth on the bed in a fairly upright recline (I tried to protest but midwife was firm and I'd been in full-on labour 36hours). Pushing took 25minutes despite my exhaustion (baby 8.3Ib) which I understand may be because SPD does some of the work opening up your pelvis... One small mercy!
Anecdotally those with SPD do seem to have faster births - not looked into any official correlation though! DD1 was classed as a precipitous labour as she came crazy fast (very fast dilation from nothing to 10cm in under an hour - the pushing was less than great due to a cervical lip however) The rest were c-sections for non-SPD reasons.
Agree with measuring the gap just in case you need an epidural. And avoid stirrups - some midwives (mine.. who was crap in other ways too) insist quite strongly so you may need to argue the point. Thankfully most midwives are not like that.
I wanted to deliver not flat on my back but didn't get much of an option. A birthing stool sounds ideal for SPD.
I'm 30 weeks with DC2 atm and getting ready to write my birth plan so all this is fairly fresh in my mind.
I had a ten minute second stage, three pushes and he was out, my (induced) Labour was 12 hours from start to finish.
The only thing I really noticed was that I couldn't lie on my back or sides during labour as it was painful (but it'd been painful to lie down for months by this point) so I spent a lot of time on the ball or sitting upright. I eventually got myself into a kneeling position with the end of the bed tilted right up to lean on for the pushing part.
This time round the pain is much worse so I'm hoping the theory that a looser pelvis means a quick delivery holds some water!
I had crazy horrible SPD with my last (first) pregnancy. I tried water birth, active birth, hands and knees, everything and the only thing which worked was the regular ole on my back, feet in stirrups (which everyone says is the worst for SPD but it worked for me).
Pushed for 2h 15m so definitely didn't mean a fast labour for me! I did it with no drugs because TBH it was less painful than the SPD ;)