Bought my first IR pack for $1 dollar two days ago here. I like it for clean to edge of breakup on up but I wish I would have bought the full pack because I'm going to end up with it anyway because of the quality of the sounds I'm getting out of it.
https://www.yorkaudio.co/product-page/mes-212-v30-limited
The absolute best and worst dollar ever spent. Best because the pack is worth 10 times what I paid for it, worst because now I have to own the rest of the collection on the site here.
https://www.yorkaudio.co/shop?page=7
I just purchased the MV30 Dual. I think this one is awesome because its well balanced and good for a variety of tones. Next on the list is to go back and grab the rest of the 1 dollar pack which I believe is the Mesa 212 V30 pack. The Mesa V30 pack is very balanced in all areas to my ears it doesn't seem to have too much of anything in the response curve. That said I can definitely see where the M20, Black Jack, and M25 packs have a woody quality to them which I love about some of my favorite guitar tones from the past like lynch, EVH, etc.
Those packs are next on the list for purchase. For the amount of money you spend on IR packs it would be short sited to not have some quality packs on hand.
In short I think that IRs are one of the key defining feature of any tone. Modelling is only half the equation.
I think the only thing that might be a diminishing return is not organizing the IR packs down to a manageable size.
This isn't really an issue with these packs because what I think you'll find is that they are organized quite well. Usually there is a range within a pack that seems to work best.
The first couple of IRs in the mix files are usually the most balanced in all frequencies and then down the list you'll find the pronounced frequencies that you might desire.