I made a quick comparison between ULTRA ENDURA and SUPRA ENDURA datasheets and came up with this:
http://sorsa-tv.ath.cx/~antalh/ultra...supra_digi.png

I matched the midtones. Highlights (low density at left) look exactly the same, and at shadows it seems that this digi paper could in fact give you a LOWER contrast and more definition in shadows. I really can't see any problem here. Spectral sensitivity curves are exactly the same. When I run out of Supra Endura, maybe I'll have to try this digi paper.