Bicycling is a very good aerobic workout for the heart. And aerobic activity is probably why your collateral's developed their capacity. So I'd think you doctor will at least want you to keep cycling. As long as the rest of your heart is healthy, I'd think that you'd be able to ride as hard as you care to.

But not being a doctor, you shouldn't trust my opinion. If I were in the same situation I'd ask several heart doctors. Particularly if your heart doctor doesn't operate as a cardiac care team as do the several heart doctors that I see on a rotational basis for regular checkups.