Does anybody know, is bsa metal insert one-piece, or it's two-pieces from each side of bb?

Yes I have the 958 in large. I have complete confidence in the frame. By the way mine weighed 2450grams in size large.

 Also I am finding with this suspension less pressure less volume. Let it sit a bit low and fill the air chamber up . I have the .6 volume reducer plus a bunch of gear grease to shrink the volume. I like the grease method as I can really fine tune it.
 Sitting at about 30% sag using this method I can pedal seated thru 1-2" roots all day with comfort and carrying speed no problem.
 But still get a nice pump and pop due to the ramp up adjusting rebound to allow this of course.

Good to hear! Also, how about mud collection, do rear bearings and shock always covered in dirt?

Looks like a regular 036 frame to me, check out this thread, there's also a bunch of build threads spanning several years. I'd look for something a bit more modern if I were you.

Thanks. I'm looking for information about this frame becouse i found it for cheap in my country, other options are too pricey for me

Hi. Do i need boost 12x148 frame for plus size hardtail or some of them can take 2.8-3.0 tire? 12x148 frames that i see are all too expensive compared to 12x142 and they are don't look nice at all .