8k miles is incredibly short for an FI bike (especially for a closed loop system like the FJ).
First off, let's talk about why you replace or service spark plugs. Two reasons. One, the spark plug physically erodes to the point where the gap is out of spec. This typically takes a really long time, and takes much longer with platinum or iridium plugs. Two, the plug accumulates carbon deposits and either fouls (bridges the gap between the electrodes, EXTREMELY unlikely in an FI engine), or builds up in the well around the insulator and changes the heat range of the plug (this would actually take, by far, the longest of the three scenarios here.
Carbon fouling is a real problem with a carbureted bike, since the air/fuel ratio will change drastically depending on altitude, humidity, etc. An open-loop FI system (no oxygen sensor) is only slightly better. A closed-loop FI system (O2 sensor in the exhaust) is much better, since the ECU gets air/fuel ratio information from the exhaust, and can change the fueling to compensate for altitude, humidity and other factors.
So really, with a bike like the FJ (closed loop FI), the primary concern is electrode erosion. Even with a standard, non-iridium NGK plug, I would expect 20-30k miles before the electrodes wear to the point that it causes a problem, and the first indicators would be skipping at idle and slightly reduced fuel economy. I feel like the manual is being way overzealous with an 8K replacement interval. Also, the nice thing is that you're really not going to break anything with a worn plug, so you don't have to worry about warranty issues if you extend the interval. Really the worst thing that could happen is the side electrode wears to the point where it melts and falls off, but that would take like 150-200k miles on a modern plug, and would be preceded by very poor running and detonation.
I'm at 8k now, and plan on balancing my throttle bodies next weekend. I'll pull the plugs while I have all that junk off and report back, but I bet they'll be just fine.