IRC 2012 code min for sub-slab insulation in zone 6 is R10 down to 4', with another R5 edge for heated slabs. MN code is based on IRC 2006, but with many modifications to the IRC about foundation insulation- I'd have to look it up.
There's a long term rationale for R10 full-coverage under slabs in zone 6 even without heating the slab (
see Table 2, p10.) With a radiant slab adding another R5 wouldn't be outrageous. If you layered 3" of Type-II EPS under an inch of XPS (better staple retention if you're stapling the tubing to the foam) you'd be at about R17.5 for about the same cost as R15 in XPS (3").
In 50 years after most of the blowing agent has escaped the XPS will have the same R/inch as EPS, and the stackup will settle in at about R17, whereas 3" of XPS would end up somewhere beteen R12.5-R13 over the long haul. If you can find a source for reclaimed roofing foam you can go north of R25 for less cost than 3" of XPS. (Stick to EPS & XPS when using reclaimed goods, and NOT polyiso, which can slowly take on water over time.)
XPS is blown with a hefty amount of HFC134a (automotive AC refrigerant) which has a huge global warming potential (about 1400x CO2), whereas EPS is blown with pentane (about 7x CO2), so whenever virgin-stock XPS is be swapped out for EPS or reclaimed goods it's a far greener approach. Low global warming blowing agents for XPS are on the market (notably HFO1234yf at about 4x CO2, under a couple of different trade names), but SFAIK none of the US vendors of XPS are using it. Unlike HFC134a the low GWP blowing agents are somewhat flammable, but contained inside the mostly closed-cell structure of XPS and under 4" of concrete there's no hazard to using it in building applications. (It's among the refrigerants approved for use in Europe, where R134a has been banned in air conditioning.)