I installed some Dynatech DC3-2 mini-coils on my 78 XS400E (US). Had to fabricate a couple of brackets to hold them but they do fit under the gas tank nicely. The spark energy was noticably better than my 30 year old coils. They are being driven from the 1980 CDI ignitor box that I installed awhile back. The coils are 3 ohm primary like the old ones and have not killed the CDI after a couple hundred miles. If you want to use new coils on your bike, this could be an option. I'm using the same resistor cap plug wires that came standard on the 78 US model. And platinum tip plugs.

http://hollymchone.com/tmppx/index.html

I believe the xs400 is TCI and not cdi, which is why 3 ohm coils work ok.

I believe the xs400 is TCI and not cdi, which is why 3 ohm coils work ok.

Yep, you're right, it is TCI. The coil spec was 3 ohm so I just matched that. I looked in the yamaha manual G model supplement and they have a block diagram showing the driver transitors inside the ignitor box.