Stat growth grows at higher levels, not diminishes.
Edit: okay I said that wrong, but the general trend is high stat growth for the first 500k XP (to make up for the lowered stats on evolution), and then it falls off for a bit. However, stat growth is more determined by the monster you are leveling, so once you find the right monster to level, you do get better gains by maxxing that monster for a team instead of spreading the love from levels 40-99 across a team of 5.
Edit 2: I made a post about it earlier in the thread
http://www.neogaf.com/forum/showthread.php?p=71242481#post71242481 for comparing 2 monsters. If you're going to spread out experience, you should be spreading it out on a team basis (picking 1 monster per team to level up) and not on a monster basis (leveling all monsters in a team equally)