If you want back to back shots you would actually want a very large Commercial Dipper where you could almost never overheat the group unless flushing in between shots for an extended time. You will pay for it though.
Izzo Alex Leva, Izzo Pompei are ideal. The dipper tube/flange on those machines are so large they actually acts as a second boiler bringing down the temp a bit as it enters the group. Profitec would require a more residential shot sequence but I bet you would be hard pressed to over stimulate it
Astoria Gloria AL1 is also large commercial dipper and can even run on Propane or Natural gas if you order it with the option or retrofit the kit. Personally I think the body is a bit rotund without the lines to make its size a true beauty.
Unfortunately for this buyer it complicates the 120 debate further as the Izzo and the Astoria are 120v 20Amp machines and will likely requite an outlet change maybe not an entirely new outlet run and electrician visit (although I've diyed that before too)
A 220v machine is ideal for power and recovery of a smaller boiler on a lever if you do a lot of milk drinks (ie. Londinium, Achille, etc) but this wont be a huge issue on the Izzo or Astoria as the boiler is so large (5l versus 2.3 or so on Londinium) you won't compromise it. If you drop the temp to match certain beans perhaps you wont want to run it for steaming in a commercial environment but it will be fine for residential unless you are steam cleaning the carpets at your house with a long attached hose