A couple of suggestions...
a) You have to determine if there is reallly any difference between the RB20 and RB 25 turbocharger. I've never seen them, so I can't say. If we assume that the differences are linear, with regards to the engine, then the RB 25 turbo would have a potential flow rate of 25% more than your current one. This means potentially 25% more air.
b) With 25% more air, the compressor wheel would have to be larger. This means more rotational inertia...slower wind-up, resulting in increased lag. You cannot avoid this unless you are willing to invest in an anti-lag system, which will halve the lifespan of your turbo.
c) Counter-balancing the slower wind-up, is a higher volume of forced air when it does wind-up...meaning higher turbo pressure potential and 25% more O2. This has to be balanced against a comparable increase in fuel. The best way would be to change the injectors to higher capacity ones (at least 25% more) and the MAF or MAP sensor to make sure that the metering of fuel is correct. Increasing your fuel pressure by about 10% will probably get you about 10% - 20% more fuel at WOT, but slight over-fuelling at idle or lower rpm's.
d) Modifying the fuel rail is optional i.e. boring it to be slightly bigger, or changing the flow to twin entry to balance the fuel flow across all the injectors.
e) To accomodate the increased fuel flow required, you will need to upgrade your fuel pump. Using an RB 25 or 26 fuel pump is an option. Or an aftermarket one like Walbro 255l racing fuel pump. A cheaper option...though not as flash...would be to put a Bosch fuel pump (something from a >3l Merc) in-line with the stock fuel pump. This is workable, but would be a tad noisy as the pump would sit under the car.
f) To take full advantage of the increased forced air flow (i.e. increase turbo pressure), it would be wise to invest in some basic strengthening of the engine. Some things to consider...
1. Slightly thicker metal head gasket to lower the compression ratio.
2. ARP head studs
3. Slightly cooler rated spark plugs (depending on what pressure you are aiming at)
g) Finally...you will need to cool the charged air a little more. Slightly larger intercooler, or, as they use in rally...fabricate an temp or rpm controlled spray bar across the intercooler to spray water into the intercooler fins when required.
Just some things you can consider...
