^Those two particular cards you're talking about have very low SPD growth if I am not mistaken.
The number of cards you need to max bonus depends solely on
1. Rarity (has more effect than most people believe)
2. Stats (supposedly after merging but needs confirmation)
3. Method of merging
4. Double EXP/Bonus thing
Or something like that.
I was gonna make a simple calculator (via Excel) on how many copies you need, but then I realized that the playerbase is not worth wasting time for.
BTW, in case anyone's waiting, I haven't checked all entries to my contest. I don't think anyone had another button I didn't know, so I'm holding another contest for another LotD. Naturally, winner can't get join.