Can't answer your question, but I was looking at the 2x 2l pots on Dobies (exact same blurb and prices).
According to James Wong dwarfing rootstock means the plant goes UGH, well I might as well put out more crops since I have to be dinky. As a result they are are higher producing in relation to size than plants with non dwarfing root stock.
If we both get one we could keep each other updated. Although latitude will affect growth. You being in London and me in N.E Scotland, your a bit warmer, but more polluted down there.