Yeah, there's an updated SMS-to-Genesis converter that was released in the UK. It looks like a plain black cartridge with a cartridge input on top and a tiny grey pause button on the front. I think there's someone selling one here on the forums right now.
It doesn't have a card/3D adapter input, however.
I actually use a Genesis 2 with the original Power Base Converter. It almost fits, but there's some extra plastic on the back that gets in the way. After taking a hacksaw to it for ten minutes, I took a different approach. I heated up a butter knife over the stove several times and melted it off, leaving a cleaner cut than the hacksaw. The video output looks great, but the sound effects are strangely louder than the music.
To partially answer your other question, the Genesis 2 has a 9-pin out with stereo sound. Apparently, the sound output is lower quality than other models and there's "Crystal Clear Audio mod" floating around the internet.