Sounds like you need to upgrade the suspension, especially if you're driving at stupid speeds like that.

i mean track
and i cannot afford any lowering springs because of the potholes

I'm inclined to agree with these folks especially if you have a lot of potholes and drive like that, track or otherwise.

The sway bar and perhaps even the braces will only further unify the rear-end, limiting the effectiveness of a fully-independent suspension. It'll work well for smooth roads, but any imperfections will cause one side to follow the jump the other side experiences.

the problem is, i'm making this high-speed turn on the track and the rear-end begin to bounce but there are no visible imperfections on the road (it's not perfect, but it is rather smooth!)

I feel, I need to stiff up something, but I cannot afford changing springs/shocks (btw, I have new struts/shocks) because of the potholes on the main roads, so there are two options for me: 6pt brace and hotchkis swaybars.

I tried to use rear M-shape brace at winter, but didn't like it - too much oversteer, my rear-end wanted to travel into the turn more than front, especially dangerous on the ice.

And I read in the first post, that this 6pt brace has "Noticeably less oversteer. This caught me by surprise at first.", so it might help?

Did you have this problem before you changed the sway bar?

^ Actually thats a valid point, i just re-read and realized you have a RWD rear sway which with the thinner X front sway could cause a bit of oversteer which seems to be the problem you are experiencing...

Well, before I got RWD rear sway, I experienced some understeer (BTW, I got front GTSpec strut brace and fiberglass underbody that works like some kind of a brace too). And after the installation I noticed that steering became more "neutural" in the turns and car feels more stable on the high-speed bumps and potholes.

I don't know, maybe you are right and it is the front bouncing on the high-speed turns on the track, while the rear-end is more solid and I should get Hotchkis sway bar set

^ those will always help, but do get your shocks/struts checked out at least at the local alignment/tire shop even if you cant afford to change em, if they are bad then thats your problem, else getting some HK sways should help clear the issue.