I've never used a CT1 but I would indeed expect an open boiler to have better temperature control than a pressurized boiler and directly connected group.
Out of curiosity, what is the max volume of a CT1 shot? I've never had a problem with shot volume on my Cremina, I can get a 40g shot without any half pulls. I wasn't aware that 40g is limiting; that's a normale even if you somehow managed to shove 20g into the basket. When I had my Zacconi Riviera however, with that thing you were certainly limited by shot volume.