I have an RB V12 Turbo (shaft and head) 3 port. Very fast motor and if operated correctly, is a much better investment than a Sirio. Similar performance (I think RB is slightly quicker and has a much better powerband across the rev range), more durable ( Sirio have a major problem with crankshaft breakages. The distributor in australia is giving away a new crankshaft with every engine because a breakage is inevitable
), and best of all, the RB importer is in Sydney and at nearly all race meets (moorebank), carries all parts, and is very supportive with tuning and any other issues you may have with the engine. I frequently visit him for help and parts and he has never let me down.
P.S. He is doing good engine mods cheap, and he covers all his work!