1-1/4" Thermopex does measure a true 1-1/4" ID, which gives you only 6 feet of head in 320 feet round trip. Add in a couple feet for the exchanger and fittings, a 15-58 will still give you 10-11 GPM on high which is more than plenty for what you're trying to accomplish, speed 2 would give roughly 7.5 GPM which is also more than most of us get. As far as the radiant goes, anything under 15 loops of 1/2" PEX can also be served by a 15-58 if properly laid out in 300ft loops or less. Any idea how long your baseboard run is? Odds are that a 15-58 would easily do that as well if it's all 3/4" like they usually are. It may be worth going to an ECM circ like an Alpha to save some electricity, but the initial cost is 3x what a 15-58 costs on eBay. Realistically, if electrical usage is a concern, ECM circs and 24V zone valves offer a considerable power savings over the regular circs. I used to be a hardcore zone circ kinda guy and sometimes still am, but am coming around to the idea of using zone valves as they're not such a piece of crap as they used to be anymore. If power consumption is not an issue, I'd go ahead and buy a stack of 15-58FC pumps from eBay