Nah, it's not the same as a 12 inch speaker. I wouldn't suggest trying to play in a band setting with the 2 internal speakers. I used one for a while sometime in the 90's - it is pretty cool sounding into a cab.
I ran mine into a 4 x 10 cab split for stereo. It was loud enough for about 99% of our gigs. At one place in Vegas, the stage volume was so freakin' loud, I swear I was playing air guitar all night, and for the first time ever the sound guy told me to turn up!
When we got home, that week I bought a 5150 combo. I think it was 1995 or 1996.
PS: I think the speakers are actually smaller than 6". They say "100 watts", but really, no freakin' way. If you search for a used one get the Mark II, the Mark I's really suck.