I suspect since the collapse of ProLeague and the collapse of the KESPA teams (with Jin Air the only team left), the practice environment for Koreans is not as it once was during the ProLeague and KESPA days.
On the other hand, the WCS region lock has done wonders for the foreign scene. I suspect the foreign pros are getting more chances to get prize money from tournaments, and this is motivating them to practice harder. At the same time, in the Europe scene, there are new up and coming players like Reynor and Clem, adding to the competitive environment of the Europe scene. Conversely, I don't know of any young and upcoming Korean players.
In cross tournaments that involve Koreans and foreigners (especially European players), the foreigners are performing quite well against the Koreans. You have HSC where the Koreans lost to many European players at HSC, although the finalists were Soo and Innovation. Scarlett won IEM Pyeongchang, denying sOs in the finals, while Elazer denied Zest in the same tournament. Then Scarlett made it as far as the Round of 8 in the GSL, which hasn't been done by a foreigner since Naniwa I think. In this season's GSL, you have Neeb and Reynor making it to the Round of 16, and Special came close. And in online tournaments, you see the Koreans losing to foreigners every now and then (although Koreans tend to win more in online tournaments). And of course, there were some upsets by European players at GSL vs The World.
I suspect the gap will continue to close because some of the top Korean pros now are getting older. For example, Classic is a few months from turning to 27 years of age. And there doesn't seem to be any new Korean youngsters coming through.
This isn't necessarily a bad thing if the gap is closing because the viewership has gone up for SC2 this year. If the closing gap between foreigners and Korea leads to more viewers, and therefore more sponsors and money injected into the SC2 scene, then all the better for the SC2 pro scene. So I am in favor of the region lock, for the overall health of professional SC2.
Edit: Serral had this to say about the EU server:
I think you can get pretty good with European practice, especially against Zergs and Protoss players. With the practice I’m getting against them, I can beat anyone. We’re lacking a bit in Terran players—we have some, uThermal and souL—but not too many. So I have been mainly just theorycrafting and thinking about how to play the game, ‘what are the correct things to do?’ and watching the Korean games.