Most of the votes have been counted and those that haven't been won't impact the final percentages in all but a few races. I took the 328 races that had one Republican and one Democrat in 2012 and 2014 and compared them to figure out the change. By eliminating the districts where one party didn't have a candidate in either year, it'll be an apples to apples comparison. Here's the spreadsheet.
Republicans improved by 3.5% in the two party vote or a 7% better margin. The GOP lost 2012 by 1.4% and won 2014 by 5.6%. I sorted it by margin change so you can easily pick out the biggest under and overachievers compared to 2012. I haven't done a deep dive, but the easiest thing to notice is that 5 of the 6 biggest gainers were in New York. What's interesting is that Republicans were only 4-1 in those 5 districts and they only include 2 of the 3 districts the GOP flipped. It's unsurprising that some of the biggest gains were made by either candidates who were defending seats they won for the first time in 2012 and ones that were open in 2014.