DD
Here's my $0.02 in hopes that this puts you at ease

I too have a 25/2 w/15KW head - Best think I could have ever bought
My wife too was totally irrate for me spending so much $$ on such a project as well as being jelous of me spending more time with Big Green - Too Bad, So Sad. One day she will thank me
I too have concerns with underloading - I have 2kw of lights and 3kw of space heaters for extra load when needed
I live in hurricane country and needed a dependable backup system that would power the whole house 24/7 without the family crying about not being able to do this or why can't I do that now - 10kw will fill the bill
Hurricane Ike comes along and wife was THRILLED that I did all this for HER. Kids were just glab they had tv and didn't have to live in the dark - Big Green ran like a clock
In summary: Enjoy your hobby, wife will thank you for doing this for her when the power goes out
Things to do:
Buy the CD
Adjust the spill timing - mine was a bear to start until I adjusted the timing. The injectors fired too soon
Change the spring - my governor and linkage was sloppy and didn't respond well to small load changes. A new spring and removing the linkage slop helped. I can go from 62 hz no-load to 58 hz full-load now. I have mine adjusted to 60 hz with both a/c's, water pump and a few lights on. It will swing between 61 and 59 hz with items cycling, being tuned on and off, etc...
Devise resistive loads and operate it at slower speeds to find its "sweet spot", where it sounds happy and has minumal vibration. Make sure you are at least 50% load here. Measure that rpm and have George (or whomever) make a pulley for that speed. You will find it to be happy ~750 to 800 rpm.
Oh, and Have Fun
Dave