At first yeah there is going to be the major risk of a brain drain, but as the Russians rebuild they will tighten border security to stop it, look at East Germany. Before the wall they had a major issue with a brain drain, but once the wall when up, very few got out. IIRC the border guards who got out generally shot their battle buddy just so they could make it after the wall when up.
It isn't the brain drain that is the biggest concern; with the Conservatives winning, the IEF's focus becomes Russia first. The border regions lost do not affect the IEF's projection and economy too much (Poland was the biggest loss, and the Baltic states do bottleneck the IEF into St. Petersburg, but I don't think they are a major contributor to the IEF's economy.) The loss of Riga, among other ports, might be a bigger loss.
The larger issue will be Russians that reside in the countries that have since left the IEF, which will create a problem rather similar to the OTL situation with Russian population outside of Russia proper. This problem will be exacerbated by the success of the Russian state (later the IEF), and how it did not experience multiple wars that killed tens of millions of its population and forced the early flight of thousands. This IEF has not experienced the turmoil that plagued Russia/the Soviet Union in OTL, and as such will likely have a larger population... Correspondingly, the Russian population in the various federal states will likely be larger. That could lead to a few issues.
Conservative sympathizers will end up fleeing into Russia, where they may be encouraged to settle in troublesome regions (Central Asia in particular; see the demographics of Kazakhstan at the end of the Soviet Union and compare). Reformers will likely flee to rebellious states, but they may find themselves pariahs; even if they supported inclusion, they will not be welcome in nation-states that have been established.
Once Russia gets its feet back under itself, I find it likely that they will begin encouraging Russians to emigrate from the west (along with any sympathizers), where they will be paid to settle in the east. I assume this for a few reasons.
There are three main sets of rebellious states: The European ones (Poland/Lithuania/Latvia/Estonia/Finland), the Central Asian rebellious states, and Manchuria. The former are under to protection of Germany, and the various power blocs would look down upon IEF attempts to recapture. The ones in Central Asia have not established anything more than preliminary government and, as Russia finishes its job in the Caucuses and moves further east, will likely be reintegrated. The necessity on shoring up the loyalty of those Central Asian states, along with the large Russian population already there, will see a relocation funneled eastward.
That leaves two problems: Manchuria and the European rebels. The latter are, again, under the protection of the AES (Germany); every other power bloc will likely affirm that. The former, however, has been usurped by a pariah government that has alienated nearly every single possible major ally and acts alone. By the time the IEF has settled problems in the west, the focus will turn further east against this one nation that has subverted its most valuable federal subject and which every other nation bloc would agree it has done. Neither the Commonwealth nor the Turin Pact nor the AES nor the LAR or any other will stop the IEF from taking action against the Technate.
The next war for the IEF won't be in the west; it will be in Manchuria. And it's likely that it will go nuclear.