Did it last year in early May - it rained torrentially for two days up the west coast, then had a couple of days of sun, then rain again for the east coast. That being said, it was beautiful riding and I have no regrets.
One thing I would change is to skip out the east coast entirely. Most of the riding is on the A9 which is utter crap, and the scenery doesn't compare to the west. If I was doing it again, I'd probably cut the top right corner out and take the road from Tongue to Lairg instead - barely any civilisation and singletrack road for miles and miles, according to some motorcyclists we met. This would give you more time to piss around on the west coast too, there's loads of little day trips you could do if you wanted to see more of it, like exploring Assynt or taking the ferry across to Cape Wrath.