Will the lower level PvP really be the same old mons, under powered?

Depends on if the system is the same as what we're used to, and if it's DPS or DOT centric. Either way, we'll likely see moveset/typing as king, meaning counter-typing will be key.

We'll have to do some work to figure out what stat shapes are best for PvP, which attacks and resistances are the most effective, but I expect this to open the door to a lot of other mon being meta-relevant, as well as vastly diminishing the importance of IVs (for the CP capped leagues, at least).

Do pokemon of the same CP perform at the same degree (considering the differences in stats between pokemon) even if they have different level?
As an (odd) example; would an 1500cp Ivysaur with razor leaf solarbeam perform the same as a 1500cp Venusaur with the same moveset? they have a similar stat proportion so the influences of their stats on the cp should be similar too.

If so that means that a 1500cp meteor mash metagross has the highest neutral dps at great league, and im debating if i should evolve some low cp community day pokemon to use in the future.