Apart Mamba kits and similars, and apart all the minor mods you mentioned, the new fashion is modifying the piston as shown in the link below.
You carve a "niche" in the body of the piston, fill it with oil and seal it with two orings: this way the piston runs smoother and faster.
I read about this kind of mod some months ago on an italian magazine. Now I see it again in the website of a new DB member from Italy, whose nickname is Hfprosoft.
The mod does not seem easy for DIY, however that's it. I'll see if I can find that old magazine and post some more details soon as possible.
Meanwhile, here's the link to Hfprosoft's website. Text in italian, but the picture somehow shos what I'm talking about
:
X-Power