Look at this size comparison:
What roles do you guys see the two ships?
(M) - System defense and Heavy Escort. The Idris isn't large enough and doesn't have enough firepower to be an assault ship. It isn't large enough to be a Capital ship for the UEE, maybe for (Non-Player) smaller military factions and Corporations. It might be used in large numbers for system defense. It also might be used as a Heavy escort for capital ships, it also might be used as armored transport.
(P) - Capital Ship, Light Assault Ship, Interceptor and Armored Transport. It depends on who is using it, but this ship will be used by smaller factions as a Capital Ship. It might be used by pirates or mercenaries as a Light Assault ship against lone targets and also might be used as Interceptor (Because it's jump capable) and Armored transport for High Value Cargo.
How much of impact will the railgun have over not having?
It depends on how powerful that gun is, but it's still a downside of the P variant since less firepower means less protection against pirates and rival companies.
How many people you think will upgrade to the (M) when they sell the kits for the (P)?
I don't really think they will sell kits for the P.