The guy at my beach who sold my 10m soul to me (and now has a 10m Roam) said that the soul had much better low end. He didn't specify a value as locally 8-9 knots tends to be the point where the wind does funny things at our beach (might shut off - might change direction - everyone comes in). In the link above, Rayner compares the 9m to the 10m soul so I would expect the 9m to have better low end than your 10m Roam and then you can size up from there to chase the extra margins as you like. In reality you might find as much low end performance increase by changing the wings on your foil when its really low wind (if you look at the wingfoil forums, those guys are riding in 8 knots on big wings ).
I think your question should be more - whats missing from the performance of your Roam as the handling is going to be so different (the Peak would be closer in characteristics to the Roam). I gather the Aluula Roams are great products and I see this one used on a twin tip boosting pretty high.
For myself, I'm interested in the extra depower. The pull through the loop of the Soul is a little stronger than I'd really like and this is something that the Falcon is supposed to be much better at than on the soul.