This is normal behaviour. Idle perfcap actually means your GPU performance is capped because it's simply doing nothing; if it were at maximum power you'd see the power consumption figures rise to 100%. I don't use Afterburner so I can't say whether you could increase the power limits in that software but I suppose it ultimately depends on the flavour of GPU you have. If your 1070 had a voltage reliability perfcap, it's most likely because it believed it couldn't automatically boost clocks any further without becoming unstable, which is down to everyone's second favourite lottery.