Hm...that is a somwwhat weird approach Would love to know why? It's customary to have a brand free section regardless of specific starting platform. Unless it's some weird business arragment...

Welcome and hoping to see some of your images!

Nicely caught

Hello and welcome to the forum

Indeed. Love the third capture. It's the ultimate 'street' shot if you please.

Nicely done. They are such wonderful birds.

Irix costs just over 400 quid for the Firefly. Distortion wise, it performs way better then the DFA 15-30mm at 15mm and I believe better then the Samyang as well. You can quite easily look through reviews for the numbers if it's that important to you. Not quite sure where you read negative reviews of it?
Either way, both lenses are fantastic value for money. Easiest thing is maybe to order one, have a play and decide if it's worth keeping or find a store that will let you fire some test shots?

I personally have the Irix 15mm Firefly and love it. The build quality, the handling, the focus lock (such a brilliant idea!). At this focal length, you practically do not need to touch the focus. Mine is set and locked on the spot on infinity mark and most times I forget the focus ring is even there; unless I am trying to be weirdly creative

This lens is a little marvel imo. The colors are fantastic, distortion nearly non existent. The only 'issue' it has, is it flares massively if you have the source of light directly in the centre of your frame. The moment you shift slightly off centre it's manageable as you can see on the photos and the star burst it produces is lovely!

Ah, the little baggy/pouch it comes with is not very convinient to use and only offers dust protection. I actually purchased the hard shell case for it which is great.

Some pics:

That's what I thought. Also, I read a comparison somewhere between using DNG and Pentax RAW format for PixelShift and the conclusion was that it's optimal to use Pentax RAW; granted, it may just be lucky coincidence or opinion based on a small sample but may be worth a shot?

Interesting. Bit hard to judge as the non PS one appears to be lighter.

Given how the moon constantly 'moves', have you thought of actually using f5.6 - f8 and a higher shutter speed to compensate a bit more? From my experience there is absolutely no visible benefit of using anything higher then f8 unless a particular lens is known not to perform too well at certain apertures but with your beauty, that is not an issue I'd think the quicker the pixel shift magic happens the better? I have been contemplating renting that lens for couple of days just to shoot the moon in PSR with the purpose of further stacking a dozen or two of exposures. Simply curious how much detail one can get that way.