I discussed this very subject with Joe only the other day (though I don't recall which injectors he was asking about, the impedances were very similar). He had come across a car with 12 ohm impedance which had been running for some time without issue.
Here's the (electronics engineer hat on) deal.
1) current through a circuit is proportional to the loop resistance of the circuit - in this case, the current therefore would increase by about 33%.
2) power dissipated in a circuit is equal to the voltage across that circuit multiplied by the current - so the power dissipated by each injector will be around 16W, most of which is absorbed through the manifold and the fuel flowing through the injector. We can assume that Lucas are happy with that sort of load.
3) the injectors are switched on using a transistor in series with the injector coil. That transistor - one per injector - is in the ECU housing. The transistor is always either off (and dissipating no power) or hard on, with a voltage drop of about 0.2v (guessing here from 1990's power transistor spec; I don't know which transistor is actually used) and so when it's turned on, the power dissipated by the transistor's internal resistance will be around 200mW; no great problem as it's bolted on to a heat sink on the case of the ALU.
4) the bad news: between those two extremes, the transistor is either switching on or off, and it can't do that instantaneously. The time it takes to do that depends on the transistor characteristics (which I don't know), the circuit configuration (which I don't know) and the speed the engine is running. While it's switching, it's not a switch, it's another resistor in the circuit and it's dissipating like mad. As the engine runs faster, the proportion of the time that it spends in this switching time vs either on or off increases, so the worst time is when you're running maximum revs - where each transistor would be dissipating around 33% more than the ECU was designed to...
Ok, we do have a couple of points: first that Joe knows of at least one case where they have been successfully run for some time (I don't know how long). Secondly, no designer worth his salt would use transistors running particularly close to their limits, particularly where the environment in which they would operate is so harsh, and where they're expected to work for decades. Personally, I'd spec at least two hundred percent overload, and try and get five hundred past the production engineers (they're more expensive!).
So, my *BEST GUESS*, no guarantees, is that at normal speeds there will be no issues. I'd check the ECU after a fast run with the correct injectors and see if it got too hot to touch; if it does, I would be very wary of changing to those lower impedance injectors. If it feels ok, change them and then do the same test. 'Too hot to touch' is about 60C at the heatsink, so the transistor cores will be at maybe 80 or 90C. Most electronics are specified for 0-70C, those for cars from -20 to +85. Running components at elevated temperature degrades their operation... make you own mind up
p.s. Injectors with impedances in the 10-14 ohm range are referred to as 'high impedance'. Don't even think about fitting 'low impedance' injectors with an impedance of around an ohm.