All forums are slow anymore. Nothing against SSA, but most traffic has moved to social media platforms.
I don't have any personal experience with the nightshade or sp4 so I can't really comment on them specifically.
Overall response though is greatly affected by enclosure design. Generally with ported enclosures (4th order vented) going larger with a lower tuning frequency will yield a smoother response than smaller with a higher tuning frequency (think peaky one note wonder)
I would suggest taking some really good measurements to figure out your max dimensions and then look at quality reputable brands for a driver that is designed to work in those dimensions. Remember you need to look at net space after displacement of driver/bracing/port. That being said, would you have the ability to run an external aero port under the other side of the 60/40 bench to free up space?
Amount of usable space will determine driver/size. Then work on power.