What could be the disadvantage? What harm could a spark plug do? Cracked block? Molten piston-crown? Blown engine? Broken connecting rod? None of the above! Experiment man experiment. Experiment with things to increase your knowledge.
No manufacturer has all the time/budget to validate every replacement part available in the market. They only check 2 or 3 models from their suppliers and that's all. Considering Bosch is a German manufacturer how come Bosch parts could be recommended by a Japanese firm whereas there are hundreds of Japanese firms ready to cater to them? That's why only NGK and Denso parts are recommended for Japanese vehicles. As for using non-recommended replacement parts, it can go both ways: parts that are not recommended can do harm and can prove better.
I used Bosch Super 4 FR78X in FX (F8B) for 55,000 km. Proved to be very good. No starting problems etc. as some people tell. One thing I especially noted was that plug-fouling was very difficult. (If you don't know about plug-fouling, leave it; most probably you've never faced it). Fuel consumption also decreased. Better cold weather starting on CNG. Engine was smoother on lower RPMs and a leaner mix could be run so idle RPM (could be) decreased. Now FX doesn't have a tachometer so can't give a numerical figure.
Then got one of my friends to install Bosch Super 4 WR78 in their Cultus G10. Been more than 17 months and no complaint.
Currently using FR78X in Bolan (F8B) for around 14,000 km. Still going strong.
Now as per this diagram, Bosch FR78X or WR78 is equivalent or hotter than your standard NGK BP6ES (more like BP5ES). (In fact it should have been colder considering it is enthusiast category). But since it runs good, I don't care. Most probably all those benefits I got was actually because of being hotter. Anyhow one heat range is not that big, so this paragraph was not needed. I put it in just in case somebody came and started ranting about heat ranges and all that.
Verdict: Go for it. Having 50,000 km peace of mind and not having to change every 10,000 km is a boon in itself.
Note: The only difference b/w FR78X and WR78 is that size of spanner is smaller for FR78X. You can use any of them.
Yeah, I whole-heartedly agree. (It was so hilarious I couldn't stop).