Gidday I have not changed the turbo on my 2.8td, but I have increased the power by having some of the internals of the injection pump modified. The stock pump comes with a component called a plunger which is 9mm in diameter. The diameter of the plunger effects the maximum amount of fuel which is put into the cylinder during combustion. I had a diesel pump specialist install an 11mm plunger from an isuzu. This along with an intercooler gave me substantial power gains over the stock arrangement. I removed and fitted the injection pump myself and the job cost me about $800 New Zealand all done. You have to fiddle a bit to get the set up right and I advise a pyrometer placed forward of the turbo to keep a close eye on exhaust gas temperatures. The turbo itself was boosting at about 11psi stock, although the RHB5 can actually deliver up to 25 psi without much issue. I would not go much above 15psi, and then only under expert guidance. The restriction on power from the 2.8td is mostly to do with a lack of fuel as opposed to lack of ability from the turbo.

Cheers