Maximum pressure that can be produced by a combustion reaction
According to New Energetic Materials, table 1.2, a list of explosives ranges from 19-48 GPa in pressure achieved.  This is two orders of magnitude greater than the 60,000 psi figure. 

As far as maximum theoretical pressure, the question isn't really clear whether the maximum absolute pressure at which combustion can occur or the maximum change in pressure due to combustion is the value of interest.  The answer could be related to the maximum pressure at which chemical bonds exist.