Shulgin states that if you were to inject 5-MeO-DMT into your substrate your mushrooms would contain 4,5-HO-MeO-DMT which isn't scheduled and virtually unknown. He's saying the 4-hydroxy will grab onto whatever you stick in there. So if you inject DMT you would get 4-HO-DMT which is psilocin. So yes spice infused mushrooms would most likely be stronger. Shulgin also says though that DPT or DIPT could be used as well resulting in mushrooms that contained 4-HO-DPT or 4-HO-DIPT.
I would love to see someone grow some mushrooms that contained 4-HO-DPT, 4-HO-DIPT, 4,5-HO-MeO-DMT, psilocin, and psilocybin all in one mushroom. Mushrooms containing just 4,5-HO-MeO-DMT would be legal as 4,5-HO-MeO-DMT isn't a scheduled drug.