If anyone wanted to count the heavy bolters on a bastion as their rough Xenos equivalent (shuriken cannon, splinter cannon, big shoota etc.) and had converted it I certainly would not complain. It may be true that some of those are slightly preferable to others but they're close enough that it will probably never make a big difference in the overall power of the thing.
You can't really do the same with the quad-gun or Icarus lascannon, but just converting some big guns that look appropriate should work in most cases (e.g. the pulse laser from a Falcon would make a good Eldar Icarus lascannon, though I guess any mounted bright lance would do).