Does the stat say 'maxed' or anything when it's done being maxed out, or just the plus/background? If you have say, a 5 star, and merge a 3 or 4 star into it, do you get to keep the 5 star while doing this or?
Basically at Lv40, every character has a total base stat total and average stat total for all their stats.
For example a typical Ryoma's average stats is this
HP/AT/SP/DF/RS
41/50/35/27/20
At Lv40 you will always hit a majority of those stats and will always add up to his base stat total of 173.
However, some units have different stat parameters that alter 2 or more stats. But your stat totals will always hit their total.
My Ryoma is a +3 Speed/+1 Res/-4 Def unit.
41/50/38/24/21
His base stat total is still 173, it's just distributed differently. Most characters will have a +3/-3 distribution for two stats, or a neutral nature. In other rare cases, you might have a +4/-4 or have variations in multiple stats like +2/+2/-2/-2 in 4 stats.
You will always hit your base stat total though at Level 40 despite the stat variations.
The great thing about merging is that on top of extra SP, you can boost your maximum base stat total. Every +1 character you merge into another, your character gains +1 to 2 of their stats.
Every merge would bring their total base stat total up by 2. If you merge 10 characters, you get a total base stat increase of 20, and +4 to every single one of your stats including your nature.
So for example, my Ryoma if I merged my spare one into him his stats would be
42/50/39/23/21
He would gain +1 HP and +1 Speed, and his base stat total would be 175 instead of 173. He would also have a Lv40+1 next to his name to show that he was merged.
If her were to get a +10, his stats would total up to 183, and would look like this
45/54/42/27/25
That's a huge increase and devastate any kind of traditional check or counter to him because of the raw increase in stats.
However getting 10 duplicates of a single character is ridiculously hard because of draw rates and only whales do it.
It's really important to merge characters to reach a base stat total increase, but that increase is usually only noticeable after the +4 mark or so. A +1 helps in very close stat situations, but aren't you aren't going to dunk anyone anytime soon.
So merging is useful, at a certain point. Other times it's just as useful to run two duplicate characters on the same team. My double Ryoma team is really good because of that.
So my advice stands, hold onto them instead of reducing right away. Don't merge unless you legitimately have no need for them. Feathers aren't difficult to come by if you can rank every week.