I cannot comment on coverage (ie Radio), but I doubt that Merlin will provide anything to improve that. To my understanding router coverage (i.e. radio coverage) functions are limited by hardware and chipset. If I'm wrong, someone will correct me here.
For speed, I don't know speed you specifically mean so will assume user experience of internet access speed i.e. internet connectivity. For that Merlin provides Cake QoS which provide an improved internet experience (when there is any contention) for your wifi and ethernet connected users, thanks to its better mechanisms for avoiding any single device or flow from hogging the outbound bandwidth.