12:1 rising rate? I'm going to have to disagree here due to the laws of diminishing returns in regards to fuel volume vs fuel psi.
6psi @ 12:1 = an additional 72 psi on top of the stock fuel pressure. Putting you at 100psi or more. Thats more than enough fuel pressure to choke off the fuel pump's volume capacity and cause major problems with leaning the engine out.
For instance, say your using a walbro 190lph pump (58gal/hr). At 43 psi it's capable of supplying 39gal/hr, but at 100psi it'll have pressure but will only be capable of forcing less than 1gal/hr through the system.
Ref:
Walbro In-tank, high-pressure Fuel Pump Specs
From personal experience dealing with both turbo & supercharger systems I'd recommend sizing up the proper injectors and tuning. Both of which will save you massive headaches and should net more power with way more reliability if you have a good tuner. Low boost (or even higher boost) and high compression isn't in the dark ages anymore, it's more about the tuning and balancing the whole setup to get it all working right. So you get a responsive engine off the boost, and the extra oomph that makes you
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Matter a fact, on all the jackson racing s/c systems (4-5 that I've dealt with) and a couple turbo kits... the single best thing we did on these for driveability & power was to throw away the FMU and go from the 240cc injectors to 310-330cc. On the FMU they lacked power and did not run right at all. And thats without changing anything on the ecu.
The ecu doesn't care how much power the motor produces, it only looks for the proper a/f and tries to 'trim' (autotune) within the allowable limits of the factory software (up to 20% fuel trim usally). As long as the ecu 'sees' the correct resistance injectors and that it can adapt the stock maps based on the o2 sensor readings it will be fine. Or so we found with up to 10psi on the jackson racing s/c kits.
Granted it wont be perfect, but you'd be suprised sometimes what modern ecu's can cope with. As long as it's not a crazy/excessive setup it can probably be gotten away with. It will just take some driving to get the ecu to 'learn' the engine and alter itself from it's preset factory basemaps.
Once we got into some minor tuning it really opened it up...
Calculators and info aplenty on how to select injectors here:
RC Fuel Injection
IF your looking for 250 crank hp on a mildly boosted setup, you should need injectors around the 500cc mark... IF the following is true:
Desired HP = 250
# of inj. = 4
BSFC = .60
Duty cycle = 80%
Fuel pressure = 43.5
IMO a rising rate FMU is simply a bandaid for a poorly designed fuel system... one that should be torn off and thrown away. 1:1 is where I think it should be, with proper size injectors and tuning. 1:1 keeps the dynamic fuel pressure in relation to air pressure in the intake the same to keep the injectors firing consistently and making it easier to tune, if you go that route. I'm not 100% sure, but there are a lot of OEM fpr's that are 1:1... (I think thats the way it is for most N/A engines - but don't quote me on that
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Also, another thing to consider is that with boost you may want to keep a close eye on the sparkplugs. The more power (ie air/fuel) made, the hotter the combustion chamber gets. So you most likely with a 50+ hp gain will want to go atleast one stage, if not 2 stages colder, depending on your driving style and how heavy your lead foot is.
Andrew