In my life, I've had three SQ decoders. Well... four if you consider the on-board giblets in my QX-9900. But the outboards were (in order): a Lafayette SQ-L, then a Lafayette SQ-W, then a Tate 101a. The on-board Pioneer stuff doesn't do much. I'm not even sure it works. It does change the sound in the room but, there's no discrete placement of instruments or even the doubled vocals on "Indian Reservation" don't appear in the front channels. They are still quite blurred in the center somewhat.
The SQ-L did better at eliminating vocals from the rear channels than the SQ-W, but the SQ-W was much better at putting instruments in the corners. Back separation was quite good. If something was to appear in FL or FR, you would NOT hear those parts in BL or BR. But there was a lot more (I believe) unintentional back center bleed. Usually drums, bass and vocals.
I'm not 100% sure my Tate is working properly. Since I've had it hooked up, I play with it from time to time but the few SQ discs I have still don't sound like their Q8 counterpart. But it pumps dramatically and everything seems to hinge on whether it can keep the yellow "balance" light lit. If any front center information goes away, Back Right gets much louder than the other channels. And if you're familiar with a Tate, and how often that yellow light flickers, you can imagine how weird it all sounds.