I have owned 3 FT-847s in the past, and have always carried out the PA10 modification removing one capacitor.
I have just bought an un-modified FT-858, a 2004 C2 version, and decided I would compare that with the mod described by G4FUF, simply inserting a slug in a coil.
Before any changes were made, I had
6m 99W @ 15.8A
4m 10W @ 14.4A
After the slug in coil mod, I had
6m 99W @ 14.3A
4m 60W @ 14.0A
The leaving the slug in the coil, and removing the capacitor, I had
6m 99W @ 13.8A
4m 62W @ 13.9A
Then removing the slug from the coil, and only having the capacitor removed, I had
6m 102W @ 15.7A
4m 80W @ 14.4A
All the readings were taken with an external meter.
The meter on the radio, in the last example with only the capacitor removed, reads 100W on 6m and 95W on 4m.
The slug in coil mod is simple to do, within the capabilities of most practical radio amateurs. Removing the capacitor needs more skills and experience, with that in mind,I would suggest the slug in coil mod for those who are not confident or experienced enough to remove the capacitor.