If you are using AnKor's latest shaders, and you're not seeing enough gloss or specular reflections on the Yak-15 models you can make a couple of edits to customize them.
In the m3d file the first line in the section that assigns the textures to the model can be changed to look like this.
(You'll need a hex editor like XVI32
http://www.chmaas.handshake.de/delphi/freeware/xvi32/xvi32.htm)
00 00
FF 00 FF FF FF FF 00 00 00 FF
33 33 33 FF
00 00 7A 43 00 00
FF FF 01 00 FF FF FF FF FF FF FF FF FF FF
An explanation for each of these values can be found here.
http://www.sim-outhouse.com/sohforums/showthread.php?40606-Projected-knowledge-base/page3
The lightness or darkness of the _s.dds will now control the look of the finish with darker shades being more matte.
To simplify the edit you can just make the _s.dds an overall dark grey typically ranging from 8% to 16% brightness as best suits the effect you're after. (Just for fun you might also try >50% brightness and see what happens)
If you have more time just follow the texture map from the _t.dds, and make some areas lighter or darker depending on if the corresponding surface should be reflective or not.