A 1550mhz overclock is only 2.7% higher than what you have. Moving from an i5 to an i7 4790K @ 4.7Ghz will net you more in games.
Many 970 cards can't even get to 1.45Ghz.
Your expectations seem a bit unrealistic if you think you should be getting 1560-1600mhz with a non-EVGA Classified/Asus Matrix/MSI Lightning style card. The best overclock Computerbase got was
1528-1566mhz on a Galax GTX970 with a
125% power target, and that's from a whole stack of 970 cards from various manufacturers.
Also, you have to prioritize what you want and find a balance. IMO, the MSI Gaming 970 offers the best
balance of performance, overclocking, price and noise levels of any 970 card in the US. You zoomed in so much on that last 40-50mhz clocks but forgot about noise levels.
Idle
MSI Gaming 970 = 27.5 dBA
G1 Windforce 970 = 31 dBA
Load
MSI Gaming 970 = 36.5-37 dBA
G1 Windforce = 38.5-42 dBA
Their GPU temperatures are only 1*C apart.
http://www.computerbase.de/2014-10/nvidia-geforce-gtx-970-test-roundup-vergleich/4/
Unless you place no value on the noise levels, and the MSI card turning off the fans which preserves the fan bearings, I think those 2 factors matter
way more than a 50mhz extra overclock over 1.5Ghz over the 2-3 years of GPU card ownership. In the context of base clocks on a reference 970, 1.508Ghz with quiet noise levels is an excellent result. I am very surprised you are not thrilled with the card. Instead of comparing it to a max overclocked 980 that costs $515+, you should be comparing what you got from your old 7950. You are basically getting a 40-50% increase in performance, and even more at 1.5Ghz for $320 or so (- the TW3 value coupon). I wouldn't worry about the last 2-3% of overclocking performance because if you really need extra performance, you can always sell this card for $200 in 12 months and get something faster.