Or maybe it's a bug afterall, because on transfer to newer builds shadow catcher's glossiness spinner gets changed, but glossiness value in rendered picture still doesn't match. And that conversion is rather strange.
Gloss value:
OLD -> NEW
0,9 -> 0,9542
0,8 -> 0,913
0,7 -> 0,8337
0,6 -> 0,7014
0,5 -> 0,6613
0,4 -> 0,6613
0,3 -> 0,6613
0,2 -> 0,6613
0,1 -> 0,6613
0,0 -> 0,6613
Edit: tried to convert values manually by table available in converter and it works almost perfectly, so this definitely is a bug. Can do proper report on mantis if needed.