My strong instinct is that GrapheneOS's plan could be problematic:

Generally, do not to invest in features that the upstream vendor isn't fully committed to. It might work today, but tomorrow or next year, etc., it might not. They might even deploy other things that are incompatible or do other things that assume the feature is disabled. In fact, plans to do those things might be the reason they disabled MTE.

Also, never support products or projects that won't work well. When someone's Pixel 11 is slow, they will think and post that 'GrapheneOS is slow'. Blaming Google will not be very credible and will look like you're just trying to duck responsibility. Your brand's name goes whatever you release; people will associate their experience with GrapheneOS (also because GOS is the new, unconventional thing; Google is the established norm); if their experience is bad, they will associate it with GOS.