Of course, iDAC2 is better for desktop only usage, I tend to think that if one likes what they have on desktop, they may want to one day take it portable, situation in which xDSD does make a nice choice in versatility.
Yes, tested MacBook Pro as well from a friend. Yes, the sonic performance was somewhat around that of my old Clevo P775DM3-G, a laptop with an actually great on-board audio. I could use it with ~300USD Headphones and not look for improving the source as most ~300 USD Headphones or less would be advantaged more by upgrading the Headphones rather than upgrading the source.
The main areas where you can find differences: Laptop's audio lacks control and power, dynamics and lacks the vividness of a proper DAC/AMP solution. Also, at very loud volumes, PC and laptop solutions will distort much easier than a proper DAC/AMP.
Of course, one could be really happy with a laptop's audio performance if they listened really quiet, or with less expensive headphones, then, I generally recommend upgrading the headphones first.