The Factorio Wiki shows all of the improved statistics for each level. Does your added levels use the same bonuses per level, and we simply get more of them?

For reference: https://wiki.factorio.com/Quality

Most of stats go by the level and mod adds another five levels - so last level is 2x base game one.
Few other things are defined separately and they are also buffed (like miner reource usage reduction).

Could you give some raw data? What % bonus is it per level after Legendary?

It's like this:
Base game has quality levels 1-3 and 5 for Legendary. They are used as multipliers for bonuses (usually it's value * (1 + level * multiplier) for various things like health etc). Multiplier depends on the stat.
With the mod it's 1-8 and 10 for Legendary+.