Giving a Pokemon a Raise Max Level (RML) only allows them to go beyond the max level cap of 10. So, a Groudon at lvl10 without a RML and a Groudon at lvl10 with however many RML will have the same power.
Nonetheless, a Groudon (or any Pokemon with base 70 power) at level 10 will have 90 base power. If you plan to use your 20 RML on Groudon, just like any base 70 power Pokemon, it will have 140 base power at lvl30.
Hope I helped! :)