IMHO a very sensible design. The way he has cut back the chamber on the intake SSR (the port comes in from top of image) probably helps with chamber stall (when the flow takes a beating when you lift it beyond the chamber). Unlike Vizard he chooses not to cut a notch near the exhaust valve.
While I would rather fill the chamber and make it more into a design like Paul Heyden’s 1000 cc fiat that is very hard to do on a mini as you would have to extensively (spray) weld cast iron. Not an easy or cheap thing to do.
However I will try something like it on the bench using ..quick set bondo. Easy to add and remove and unlike clay/plasticine it will stay put on the flowbench. The approach with the radius as done by GR is just the same for a filled chamber so it is worth a try.
Valves and Sizes
I have a whole bunch of usable 33mm valves from various 1275 heads converted to 35.7 mm intakes and that is about as big as you can stick in there for full race use. As you can get a a +135 overbore for a 1100 long stroke crank and a +100 overbore for a 998 (well you could fit the +135 ones on a 998 but you would have to machine a lot off the top)
Bonus: they are free (Like) and you need a few extra for trying out additional back cuts. Things to try out are 15-30 on the valve and 45-60 on the seat instead of the single 30 degree on the valve.
(Generally the back-cut on the valve is 15 degrees less than the seat angle according to Darin Morgan (of Reher Morrison) up to 50 degrees seat angle.. then all bet are off.)