Ok I did a more detailed thickness calculation.
The ESBWR has a nominal wall thickness of 182 mm inc. stainless cladding. Assuming cladding of 6 mm (anyone know exact thickness?) gives 0.176 meter.
The vessel inner radius is 3.55 meter, design pressure 8.62 MPaG, giving (3.55*8.62)/0.176 = 174 MPa stress. The A508 has rupture strength of 550 MPa @ the design temperature of 316C, so this design pressure produces 0.316 of the rupture stress. So this appears to be tensile limited (not yield-limited as I had assumed).
So its a simple apples to apples comparison. Keeping the same safety factor to tensile, and using the lowest rupture stress of the reference given above for the tough maraging steel,
http://www.osti.gov/scitech/servlets/purl/4807312@ 315C, that'd be 139200 psi or, in units that make nonzero sense, 959 MPa.
So the thickness can be reduced (959/550) = 1.74x
The wall thickness can be reduced to about 104 mm (110 with stainless cladding. Cladding strength is often not credited in PV codes). The wall stress is 294 MPa (actually less because of the cladding).
ESBWR bottom vessel head thickness is 260 mm. This is cut to 150 mm.
The thickness is greater than with In718, but the alloy is cheaper and more importantly has much better toughness and easier fabrication than In718.
These thicknesses should be quite feasible for most of the larger facilities. A 4.3 meter tall ESBWR ring forging would be only ~80 tonnes. There are many forging facilities that can do this.