I don't have any experience with the A&H, but i recently bought the 1640i and i really like it. The converters sound decent for the price (i have yet to do a side by side test with my roommates fireface 800), and you can crank the pres even on something like an sm7b without any noise.
I just replaced a 1220i (same Onyx pres as the 1640) with an R16. The noise seems basically equivalent to my ears - near silent in both cases, even on quiet acoustic guitar and intimate vocal parts. I do find I run the Zed's pres at lower gain, but as I'm on the rapidly ascending part of the recording learning curve that's probably more to do with employing sensible gain-staging than the pres themselves.
Would upgrading to the onyx 1640i be a step up, preamp wise, in comparison with the 1402 vlz pro?
Absolutely. The 1640i has Mackie's Onyx pres; the 1402 has their XDR2s. The Onyxs are their top-of-the-line pres and there will be a difference ranging from minor-but-noticeable to significant depending on the rest of your chain and how you use it. The Onyx is a well-regarded, quiet, high-gain, transparent preamp. I have never used an XDR2.
Both the Zed and the 1640i are hugely more capable mixers than the 1402 you've got; you'll not be disappointed with the noise levels of either and you'll love the extra flexibility that comes with a built-in 16x16 firewire interface and on-mixer buttons for routing control. Beyond a certain, basic level of equipment the results you get are pretty much all down to you. "It's not the wand, it's the wizard," as someone said here recently. You'll be safely in that territory with the 1640 (or Zed.)
EDIT: Your main concern is noise. Either of these mixers will eliminate that problem completely: the pres on both are as near silent as makes no difference.
It is reassuring to notice that people seem to find that the preamps compare with eachother as far as noise level goes. Basically it seems to be so that if you cant make good audio recordings with either of these two boards, the problem at least isnt going to be their preamps.
Keep your opinions coming though. This is good posterity.
Maybe someone has an opinion as to how fireface 800 preamps compare to the zed r16 and mackie onyx 1640i, noise wise? And what about those of the steinberg mr816x?