I have no proof of the specifics, but i suspect the spring rate curve of the DPA differs compared to the solo air in ways that you can’t tune out with spacers.
The DPA balances the positive and negative at topout. The solo balances them when the main piston seal traverses the transfer port which is a little way into the travel. This implies the DPA will have preload that the solo air won’t have. Numbers run by someone else suggest about 5kg.
Depending on how you optimise the rest of your setup this could be a good or a bad thing. I just don’t think it is appropriate to expect 100% parity between two fundamentally different designs.
Caveats.
The interwebs will provide world views:
1.stick a coil in there; everything is better with a coil; linear innit?!
2.add bottomless tokens to get plush up top and support; progressive innit?!
FWIW,I have a DPA Pike (came with the bike) and I’ve (recently) learned how to get it to my liking: less sag; fast rebound; lots of lsc. This tamed the braking bumps in Morzine sufficiently and I was enjoying my high speed riding more than ever.
YMMV