Pex tube has both a temp and pressure rating printed on it and that will be your indicator of pending failure. House water pressure is much higher than boiler pressure. 50-60 lbs. @ 210*F+ might be a little too much for the PEX.
When it ruptures...then your boiler goes up to house pressure.
As stated by *lsayer*, PEX is not even close to copper as for heat conduction, besides, the copper coils have additional fining it's entire length to improve is abilities to conduct. Smooth PEX is a far cry, even if adding a couple extra wraps.
Before it ruptures you will have mediocre results at best.