Puma is usable, in fact it is in our Gemfile. But as it stands now you would need 2 processes, one for puma and one for sidekiq, both of which would share no memory.
So that adds up to say 400MB RSS and that is even before postgres and redis.
That said, puma would be more “laggy” due to the GIL and lack of out-of-band GC. You may get similar throughput, but time per request would vary a lot more.