Let's throw some additional data in here. Four weapons options:

1) Nothing
2) Level 0 Takenomata Kaneko
3) Level 15 Kaneko Sword (P)
4) Level 65 Water Gun

Attack the fifth opponent of the Pumpkin Dream event with an elite marksman in a solo group and check only the damage dealt to the front target.

Level 55 Uesugi Kenshin, with an inherited SR Naoe Kagetsuna healing skill and no gear equipped besides a weapon:

No weapon: 3094
Takenomata Kaneko: 5094
Kaneko Sword (P): 6294
Water Gun: 6547

Level 45 Dragon of Echigo (four stars), with no inherited general and no gear equipped besides a weapon:

No weapon: 2661
Takenomata Kaneko: 4444
Kaneko Sword (P): 5643
Water Gun: 6110

Level 55 Saitoh Dohsan, with no inherited general and no gear equipped besides a weapon:

No weapon: 2066
Takenomata Kaneko: 2259
Kaneko Sword (P): 2819
Water Gun: 4291

Level 55 Takeda Shingen of Kai, with an inherited Takeda Shingen of Kai and also having a level 65 Golden Armor, level 65 Hatazirushi, and level 20 Perfect Buddha Statue equipped:

No weapon: 9070
Takenomata Kaneko: 9475
Kaneko Sword (P): 10640
Water Gun: 13738

Both Uesugi Kenshin and Dragon of Echigo are clearly getting some large bonuses from both the Takenomata Kaneko and Kaneko Sword (P) that Takeda Shingen of Kai and Saitoh Dohsan are not. I'd be inclined to call it a flat 150% damage from the Takenomata Kaneko, not an extra 150% damage.

I'm less inclined to say that that's the case case for attack, however. The attack ratings for Dragon of Echigo are:

No weapon: 2685
Kaneko Sword (P): 4146
Water Gun: 6904

If you assume 150% attack on the Kaneko Sword (P), that puts you at 6219 attack. The attack gap between Kaneko Sword (P) and Water Gun is still larger than the damage gap. You'd expect it to be the other way around, as Water Gun also decreases the defense of the enemy. If you assume 250% attack, which is how I'd read the tooltip, you're at 10365 attack, which should be way more than a Water Gun.