Hey Chris,
I'd have no reservations about the cast manifold depending on your intended turbo choice and what you are trying to achieve. I don't think I am at "the limit" by any means. I have a Greddy intake to go onto mine and was also thinking about trying a 6766 but the car has taken a bit of a back seat in preference to working on the house lately.
I would make sure you port the manifold and open the wastegate port right up with very long smooth transitions into it from the main part of the manifold. I would also make sure you join the wastegate dump tube back into the downpipe, keep all the wastegate piping very smooth with no real turns, make sure the downpipe is 4" (and ideally the exhaust), and don't go silly on the turbo A/R - the spool will be very fast anyway so you don't need to choke things up. Back when I first did it, my 0.81 A/R 6776 was spooling faster than Jamie's 0.68 A/R 6776 (tubular manifold) and making more torque and power at the same boost.
What turbo are you planning to use and what is this for - road use, drag racing, track use etc?