Is it worth separating your vo2 max wattage's on the flat and hill climbs in order to provide clearer goals during training? My watts per kg on hill 5 minute hill climbs seems to be 30 to 40 watts higher than what I can achieve on the flat. This is where I get dropped the most often during group rides and races. 

My maximum 5 minute watts/kg on a hill climb is: 4.90 
The average gradient of the slope is: 9%


But on the flat the maximum I have managed is: 4.37


I would like to add I'm not sure how to extract the data for just the vo2max intervals or whether that would be more useful or not. 

Do I train each separately to give myself the best chance of improvement in each discipline?
TL;DR

Your output should be consistent regardless of what type of terrain you're riding on, but external factors coupled can skew the results. 



I've trained with power for many years, and while my evidence to the following points is purely empirical, maybe it will help you. 

In my experience, I've always achieved higher W/kg results on hills than riding on flat ground. I believe this is because:


More consistently positive gradient. When riding on flats, typically the ground isn't exactly flat, and will fluctuate between +/- 0.5 percent without the rider noticing. Going down a 0.5 percent might translate to the rider as a false reprieve from the effort at hand, skewing the average. As I'm sure you know, maintaining the same wattage downhill is difficult, though probably because of purely psychological reasons. 
Fewer external factors such as wind to content with. It should be noted that air resistance (drag) approximately follows a squared relationship with the free-stream velocity. Therefore, a tailwind or gust of wind while climbing 8 mph has less effect on power output than a tailwind while travelling at 25 mph. This will lend itself to more consistent power output. 
Flat and straight roads tend to have high speed limits, which means cars will be passing you faster. This wash from the cars can skew your results through the same logic as above. 


Granted, this evidence is only anecdotal but these are the assumptions I've always used to justify differing performance numbers. 

As an aside, I've looked up your W/kg on a cycling chart. 4.9 W/kg puts you at a high Cat 3, low Cat 2 range. Even at 4.37 you're still a solid Cat 3. If you're getting dropped on group rides with these kinds of numbers, I would suspect that your limiting factor isn't your 5 minute power (unless, of course, you're exclusively riding with Cat 1 riders). I would take a guess that your limiting factor isn't your power, but your ability to recover after efforts. Decreasing recovery time allows you to attack more, recover from surges at FTP, and makes your a more resilient rider. That being said, while 5 minute power is a decent gauge of fitness, the real litmus test is the 20 minute from which you can estimate your FTP. I'd be interested to see those numbers.