Thanks KFC! I actually did my play through a couple of weeks ago. I went ahead and put it on Valeria, and tried some testing on some enemies that were not weak to fire, comparing before and after damage, but since physical damage varies quite a bit in this game, unlike magical damage, it was hard for me to find out if the 5% damage boost applied or not. It definitely boosted her damage by 50% vs enemies that were weak to fire, though.
More interesting was the double beat rune. It seems to be very bugged in this game. I noticed that while it doesn't give you an extra attack on single enemies in random battles, it did grant an extra attack against Neclord and the Crystal Core when equipped to Lorelai and Milia, respectively. However, I know when Viktor had it equipped vs Ain Gide it didn't give a second attack, and I've seen videos of people who have it on Viktor during the Neclord battle and it doesn't do so either. I also know that when enemies were close in speed to Viktor in the final dungeon (specifically the Imperial soldiers), he never got a second attack in, since the game would let the mirages attack at the same time as his attack animation played. I also know that when I had it equipped to Kasim, and he attacked after a Krin with a spark rune attacked, he did not get a second attack. I have the game footage recorded, so I'll try to upload it one day so us Suikoden fanatics can see it. Haha, it just goes to show how fun it is to analyze a game that is about 20 years old and still find out about certain quirks and game mechanics.