It should be significantly better than SPDIF or HDMI with a suitable amp.
This is only true if:
a) You get rate control between the player and the receiver or pre/pro. Pio players work with Sony receivers, we know that.
But there have been other instances between different manufacturers where this is not the case.
Incidentally, flow control is automatic with Denon Link.
b) You believe that jitter is audible. The *only* difference in the data between coax/toslink and i.Link with flow control, is that jitter is minimized with flow control. Otherwise, 1's are 1's and 0's are 0's. Some jitter numbers for current players are: 3910: ~350 ps, Marantz 9600: ~350 ps, Pio 59AVi: ~250 ps, Pio 79AVi: ~150 ps. I have seen references where *some* people don't believe jitter can be audible below 1000 ps, and I've seen other references where they talk about tens of thousands of picoseconds necessary for the audibility of jitter.
I think the real benefit of i.Link isn't that the sound quality is going to be appreciably better than coax/toslink, but that you can transfer DVD-A and SACD digitally, and then get time alignment (distance compensation) and crossover filtering done in the receiver or pre/pro.