Wow, I discovered a trick with the random HP/SP giving food. For example, the Meat-Lover's Delight (XD) with the Sea King Fin, White Radish and Seagull, gives a random amount of HP. I discovered how to easily get the highest amount.
When you make the dish, instead of feeding it to a character, put it into "Store" instead and put it in your bag. Then equip it, and press the "A" Button to use it. The menu to feed the item will appear. Press the B Button if you're not satisfied with the amounts, then press the A button again. That same menu will come up but with different amounts.
I'm gonna start logging the amounts for what each item can feed (max for random, the two amounts for non random).
So far for Luffy, I haven't seen anything past +168 HP for Meat-Lover's Delight.
EDIT: And upon research, the amounts are messed up. It says its gonna add 160 but adds 150 instead.
EDIT 2: So it seems like the game likes to give a random amount than the one that is shown. Bollocks.