Since its percentage based, the difference, assuming you even can go over the cap, would be negligable.
Each point of LMC (or 5 points, if Katrena is right) should be more effective than the previous point because it is percentage based. An easy way to demonstrate this is to ask ourselves how big our 'actual' mana pool is as we increase LMC.
If we have a character with 20 LMC, then a 10 mana cost spell would be 8, or 80% of its normal mana cost (100% - 20% = 80%, duh!). If the character has 100 mana, then the character's real mana pool would be 100 / 80% (each point of mana only takes up 80% of the space a normal point of mana takes), or 125 mana.
If we have a character with 40 LMC, then a 10 mana cost spell would be 6, or 60%. A 100 mana character would have 100 / 60% mana, or 166.666666... mana.
So from 0-20 LMC, we get 25 extra mana, while from 20-40 LMC, we gain ~42 mana. Put another way, 0-20 LMC is a 25% increase in actual mana, while 20-40 LMC is a 33% increase. 40-55 LMC results in another 33% increase despite being only 15 points.
The problem is that mana costs aren't very granular and small benefits can be rounded off.
Edit: Graph
https://www.desmos.com/calculator/rkng4d6wmq -- as you can see, as LMC (x-axis) gets closer to 100, mana pool (y-axis) gets closer to infinity.