On paper, the i.MX 8M Plus has 20% better CPU performance than the Quad (1.8 vs 1.5 GHz), but the Plus has half the GPU performance of the Quad (compare the Vivante GC7000Lite vs GC7000UltraLite). The Quad’s max video hardware decoding is 4K@60 HDR whereas the Plus only supports 1080p@60. The only benchmark that I can find is Geekbench 4 SGEMM (floating point performance in GFLOPS), where the Quad scored 12.0 and the Plus scored 14.0 (which is 16.7% better).
Yes, Purism could make a phone based on the i.MX 8M Plus, but it would have to add a HDMI-to-DisplayPort converter chip so it can output DisplayPort Alt Mode over USB-C or make a phone that doesn’t support video out. The Plus has a bunch of new components (neural processing unit, image signal processor, digital signal processor, advanced 3D audio and hardware video encoding), that will require new drivers, which is why I don’t think that Purism will bother with the Plus, because it will require a lot of work for questionable benefits, so I think that Purism will decide to wait for a future i.MX 9 processor where the extra dev work is justified.