Irix 15mm filter size

So I was about to buy it when I noticed the gigantic filter size. I should add another 100 pound to buy a nd filter. Thoughts? Honestly who cares about that aperture

For a lens of that quality wit filter still cheap

Pretty much. It's a fantastic lens. Depending on what filters you need and how often you use them, you may want to consider using in body functions to replace them. If it's for regular use, then it's a no brainer to get a filter or two depending on your needs. I find grad nds nowhere near as useful nowadays as they used to be due to bracketing options. Regular NDs can also be replaced with the Interval Composite Mode in all newer bodies. That actually gives you more flexibility and generally much better quality of the end result. Only downside is higher shutter count if you actually do sell your bodies at some point. I got a 10 stop Hitech Firecrest for the Irix for about 100 quid and love the combo to the point that I am not considering getting the DFA 15-30mm anymore. Rent it one out, used it and find it offers nothing superior over the Irix at 15mm; on the contrary. Example of the combo: link

Nd filter (6-10 stops) are still essential for very long exposures

No, they safe your shutter from overuse, you can go up to equivalent of hours of exposures using the composite mode. Way over what you would be able to achieve with a 6-10 stops. I can go much beyond my 16stop ND with it; results are practically noise free and if you have an SD card large enough to safe the whole process in RAWs you get to pick and choose any stage of the exposure you see fit in case you miscalculated or the light changed in the process. I am yet to see a downside to this method outside of increased gear usage.

I haven't tried lately to use the multi exposure, but when I tested it a long time ago, I remember I could clearly notice that it was the result of stacked expositions. I will test it again in case I remember wrongly.