It will be interesting to hear how your road testing goes when the new brake linings are fitted. Your mod is the simplest way to up the leverage and with good linings it should equal the stopping power of the Vincent balance arm.
With a doubling of force every part of the system can be expected to flex more and contribute to the spongy feel. My guess is the long twin cables particularly will experience stretching of the inners and compression of the outers. This could be reduced by fitting heavier cables. I would also look at the brake lever mounting on the bars which can often be seen to flex. Not so easy to fix that other than making sure it’s tight and fitting closely to the handlebar. The worst part of all this is that it may be possible to pull the lever right back to the bar – an MOT problem, as well as a limit on the maximum braking effect. Careful attention to the adjustments will be necessary.