High THF and MeTHF would inhibit DHFR, meaning it is less able to recycle BH4.
I doubt that we can assume that high THF will reduce demand for DHF>THF conversion by the enzyme since that is more likely to be driven by DHF levels - ie by TYMS activity which produces DHF.
If the assumption did hold then there might be less demand for this aspect of enzyme activity but I have no idea if this would be sufficient to cancel out the inhibition.
You would need to study intricate details of the various enzymes involved, things like rate constants, effect of inhibitors, to begin to make an informed guess.
