Two points Chris.
1) I'm probably a big twat for repeating the claims of another person without checking their validity. My bad & you are right, the number is too high to be correct.
2) You say that you cannot change the standard computer on an SPi engine. This is not true, you can change it. What you can't do is get around the shape of the SPi manifold with its tiny little throttle valve and 90 degree bends, unless you change that as well. It is entirely possible to change the standard computer to another that will drive the single injector. There are heaps available that can do that. It would just be a wiring & reprogramming job to do it. If the injector flowed too little you could change that too. What will be the power results? Who knows but maybe not worth the effort due to the manifold's flow and over-run issues associated with wet runners.
I have read all those threads you linked and there is no one who has said that the above is not true. The discussions just centre around whether it is good to go for a non-port injection system where the fuel/air are mixed completely (in some sort of plenum or with a variable rate, non-pulsing injector) or whether it is worth the (very big) hassle to try to get a sequential system to work as efficiently as a carb set up.
It is off-topic here but multi-point, efficient port injection on a mini is a real challenge. As anyone who has gone through the siamese port theory will know, efficient port injection requires:
Sequential injector firing, one injector pulse per cylinder per 2 crank revs, careful (mappable) timing of the injector pulses to get equal A/F ratio in each cylinder, good manifold design to get the fuel into the cylinder without wetting the runners, probably more than one oxygen sensor, a computer that can run all this and finally a tuner who understands and can set it all up.
The rover MPi works-around some of these requirements by increasing the runner surface area and spraying the fuel at the large surface, evening out the A/F ratio of the charge air as it runs into the port. It is still sequential though, unlike the most cheap after market EFI computers.
There are only a few aftermarket EFI computers available in Aus that can do all the things listed. Motec high end models have sequential capability and also the ability to trim the injector pulse phasing for each injector according to certain parameters. If 4 injectors were used, 2 in each runner, then the Motec phasing adjustment may be enough to equalise the A/F ratios. Of course the EFi computer and required dyno time would be worth about as much as the car so it will take a someone with enough money and a point to prove to do it.
Summary:
SPi = limited potential but re-programmable if you want.
MPi = limited potential from Rover system and a shed-load of work for an after market system. Better off doing an engine conversion for less money and time with certain results.
Engineering is facts, what's on the internet, including this, is just some dick's opinion.
Of course I could have got the wrong end of the stick and then I'll look like a complete twat 