I wouldn't expect all that much... While CIV V has a lot of mods right now, only a handful of them go into any sort of depth beyond basic XML, and most of those have by now been abandoned. Unless an onslaught of modders previously hiding in the shadows start modding for V, I don't see this having a huge impact, especially with CIV V STILL not supporting mods in multiplayer.
Modders coming over from IV could make an impact, and while the lack of a .dll was always a main complaint as to why the switch was never made, I think a dislike for the streamlined mechanics of V, (and the lack of any sort of depth whatsoever combined with atrocious AI), have had an equal say in why not many modders have bothered up to this point.
I've the same feeling that the dislike of the game mechanism was the first reason that have made Civ4 modders stay on civ4.
The lack of DLL is important too, yes, especially for the bigger mods that had already a large part of their code made in C++, it make sense to wait for the DLL rather than converting the code to Lua and switch it back to C++ for performance when the source is released.
But to make a good mod for Civ5, you'd have to make Civ5 a good base, and it has taken 2 years to Firaxis to make it. And I know it's still debatable, so let's just say the base game is way better after 2 years and one expansion. Still not what I expected, but truly better.
That's sad, because the 'potential' is really here, IMO Civ5 without DLL is more moddable than Civ4 without DLL, it would have required a lot of source code to convert to Civ4 the WWII mod I've made for Civ5 using Lua only.
And that's a point going in your direction: the source code will not make as big an impact that it had on civ4, because there were already a lot of thing doable in Lua (a RFC conversion was possible for example)
So we'll see what's available in the source we'll get access to, but even if the first part of my post may sound pessimist, I really hope that the access to the diplomatic AI and some of the core mechanism will allow to finish to fix the base game and start "real" modding.