Not quite, unless it has changed. Here's a quote from another forum:
"Only level 20+ bosses will drop the 6* boss cards. While we can't publish the actual odds for each boss level as things are subjected to change, I can say that the current boss card drop rate per card received scales from ~0.3% on level 20 to ~15.6% on level 70 (non-linear polynomial curve, for reference lv 50 is roughly ~6%). Also, the appearance of the raid bosses are actually set fairly high right now because we want to allow high level players to be able to get past the low level bosses faster."
This was valid for the old 6*RBs. My observation of drops from lvl 70 bosses is that I get a RB card every 3rd to 5th run on average with two cards dropped, so it still seems to be unchanged.
Here's the link, if anyone is interested:
http://forums.toucharcade.com/showthrea ... 61&page=59Post #589