Does anyone know if this can be modded for higher gain like the cascade mod?
Don't know.
What is this cascade mod you're talking about?
Do you have a link to a page where it's being discussed? A drawing or explanation?

If you can supply a schematic for the MHZ15 I can tell you if Mod the could done.
But I don't think Marshall has made those public.
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The cascade mod was commonly done on earlier Marshalls with two channels. It consisted of removing two of the inputs and "cascading" the first input channel into the second before sending it further on in the amp.

I think the haze has digital channel selection so while its possible, it would require a schematic for us to be able to figure out if you can do it.