Hi James,
Yes, I have cycle toured there 3 times, 4 months in all. Full on touring, bags on bike and staying somewhere new each night. I have plenty of time, you could see much of it in a month if you wanted.
It is generally regarded as one of the easiest long haul destinations to tour. Pretty good roads, plenty of accommodation, widely available food (as long as you like local food), plenty to see, good climate, friendly locals, etc.
I have cycled most of SL and the roads you were on are probably the worst. Once away from the west coast and the roads to Kandy, traffic is very light. It always builds up in the towns with all the local traffic, but it moves very slowly. Once away from the towns, the roads quieten down again. You have to get used to the rules of the road, but generally its just give way to everything and forget your road rage.
There are plenty of lightly trafficked roads that you can find, just have a look at Google maps. I cycled right down the east coast and for much of that you hardly see any traffic.
You get to stay in small towns where you never see any other tourists, but you may have to accept lower standards of accom and only local food, which can get tedious. As a cycle tourist you get to eat a very different cuisine to the average tourist. Hence why I don't particularly rate SL food. However, as the island is pretty small you are never more than a day or two away from a tourist town.
You need to take your own bike as you can't get anything half decent there. This is no problem as you can carry it for free on some airlines eg Emirates.
There are some cycle blogs on crazyguyonabike.com - Bill Weir's write ups are pretty good.
PM me if you need any more info, but please read up first as I'm sure Bill's stuff will answer many of your questions.
Enjoy