US Marathons went to 33 pounds until 2/08, then they switched to 35 pounds. Canadian Marathons have never gone above 30 pounds I don't think.
I'd also look into the True Fit. It has a 35 pound rear facing limit in Canada and a taller shell. The tallest shell in both countries, in fact, for convertible seats.
Piper was 4y8m old and I don't know, 36/37"? maybe when she outgrew her Britax Wizard by weight (close enough, she was 31.5 pounds naked and it had a 33 pound limit). She fit rear facing into her Radian at five years and 32.5 pounds. Now at nearly seven and 40 pounds she'd still fit rear facing in a True Fit. I haven't put her in her Radian recently enough to see if she'd still fit rear facing, but it'd be pretty darn close. I'd definitely look into the True Fit, though. I don't know what the prices are like up north, but here the True Fit is $150-$190, the Radian 65 is $200, and the Marathon CS is $310. The True Fit has a seven year lifespan, 17.5" top slots and 35 pounds rear, 65 pounds forward. The Radian has an eight year lifespan, 17.5" top slots, I don't know if they're 30 or 35 pounds in Canada rear facing, and 65 pounds forward. The Marathons have just shy of 17" top slots, six year lifespan, and I think 30 pounds rear and 65 pounds forward in Canada.
Wendy