I'm about to pull the trigger on the 200, just a question, did someone paired the 200 with a 115 ext. cab?, how about it?
Nobody's given a comparison review.
Guessing is made harder by the fact that the high power rating suggests that the 115 Extension has a different driver than the one in the 200 combo. It could be more or less efficient in terms of dB per watt, so who knows...
Still, the usual factors should apply for the most part:
The stack's low end should sound deeper and richer, because the coupling effect will bump the bottom octave by about 3dB.
Overall loudness should also go up by about 4.5dB. That's partly from doubling the cone area, and partly because the amp puts out more power into a 4 ohm load.
The stack will also sound a little brighter and more detailed, and you'll "feel" the bass more, simply because the upper cab is closer to your ear.
Still, it's an expensive route, when the 500 combo is only $100 more than the 200 combo. If there's any likelihood at all you might need the extension cab at some point, I still maintain that you should start out with the 500. With 250 watts to spare for an extension cab, the 500 is simply going to drive the 115 more effectively.
Loudness-wise, the $500 Rumble 500 should be equivalent to a $700 Rumble 200/115 stack, and the $800 Rumble 500/115 stack to a $1,400 pair of Rumble 200/115 stacks. Those numbers aren't very favourable.
Granted, a double Rumble stack would look impressive, but load-in and load-out would be a pain despite the low weight. Plus, you can get into some seriously tasty rigs for $1400...