I7 7700HQ memory controller supports ram up to 2400, assuming that's the processor going by the model number. Not sure if 2666 is actually running at that speed? unless laptop supports xmp? which isn't usually the case with laptops, at least nowadays, maybe yours does. Check with cpuz anyhow under memory tab, frequency should read 1333 single data rate equalling 2666 double data rate. 2400 would read 1200 in cpuz.
As above, if the 8gb ram in it can't be replaced because it's soldered then your current ram speed is what you'll be left with, won't be able to go higher without compatibility issues. Capacity + dual channel would favor you more anyhow, even if circumstances is restricting ram speed. 2666 to 3200 isn't a great deal upgrade, another 8gb for dual channel + more ram would be.