If price isnt an issue then there are a couple of places that are worth checking out:
Il Mare - its an italian restaurant at the Mulia but they serve a steak on bone which is ~600gm (ex bone) which is done quite nicely
Another italian restaurant at the Kempinski also does a great Wagyu steak (sorry the name escapes me at the moment)
Lobo's at the Ritz Carlton does a pretty good steak as well but to be honest with you, none of the steaks Ive seen written up in here or I have mentioned are going to stack up to some of the steaks in the States or Oz. Im not sure exactly why that is, they are nice in their own right but its not quite the same, volume is part of it but there is something else I cant quite put my finger on... the other tell tale sign is that all of the steak places I have been to here (by default) provide sauces and condiments. A simple rule for me is that a good piece of meat and well cooked steak should be eaten alone, its flavours should be more than enough and a sauce only masks the true flavour of the meat so you may as well be eating cardboard with the sauce..but thats just me..
Oh there is also a steak place called Chicanes, I think, inside the mall but just around the corner from X2..that was very very good too!