First of all, welcome to Bersa Talk, zig.
A Firestorm mini 40 is manufactured by Bersa and for all intents and purposes is a Bersa. I believe all parts are interchangeable, particularly the mags. I have two Firestorm Mini 9 mags I use in my Thunder 9 Ultra Compact.
The difference in appearance is that the trigger guards are different, and a holster that fits the Firestorm might not be an exact fit for a Bersa Thunder.
I don't own a 40, so I can't address any of your questions about experience or performance in reference to the cartridge caliber. However, my BT9UC has been absolutely flawless, and is of virtually the same design. I would expect a Mini 40 to be of the same high quality and to perform to the same standard. I haven't noticed any complaints about it on Bersa Talk. You could do a search and see what you get. I bet you won't find much.
There may have been issues with the early production, but I don't know.
I assume you're looking at a used gun. If it was NIB, I wouldn't have any qualms about buying, providing the price is not unreasonable.
I own a Firestorm 22, and it's like a twin brother of my Bersa Thunder 380, with the only visible difference being the trigger guard, the rubber grip and the caliber. It's a fine little pistol and a lot of fun to shoot. I can vouch for the quality of Firestorm. It's the same quality gun, made in the same factory as the Bersa Thunder.
p-t-max
I'm no expert, I'm just, well---opinionated!
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