I would generally agree with mobo2112 on temp ranges and that I've done runs in a 200 weight. Though I'll add that ran a marathon in an Icebreaker competitor Smartwool Midweight which I'd place right between 200 and 260, like the 220 GT maybe, at 45 degrees and I was absolutely fine. The thing is merino allows sweat to hang around long enough to get the evaporative (word?) cooling effect in either brand shirt. Synthetic can pull the sweat away too quickly. In fact I'd say the only thing to keep in mind is that as soft as merino can feel, on longer runs it can beat up the nips.

Ironically, I'm going out tomorrow morning in a 200wt icebreaker mondo (no outer layer). Conditions will be dark (530 am), 40deg, feel like 36. I'm doing 6M tempo run, slower than race pace but definitely hard. I am expecting for there to be some sweat, expecially at the elbow crease, and lower back. I could go with a lighter synthetic top, but i like the insulative/warmth of wool...even when wet. The zip will dump some heat, but i'll still sweat. If its going to be colder, then i would go for a synthetic t underneath, a very light jacket, or light synthetic top over my mondo vs going w a heavier wt layer. Assuming your hard runs are in the cold (30's or less), i think the 260 is on the cusp of being too heavy. If your talking 40's or more, than i definitely think you can go with a lighter top. If it were more of a steady run, then the 260 would probably be fine as you wont be pushing your heart rate too high.