yeah i think to 15 feet you should be alright but you will have more plugs up and suction will back of a lot at those depths id say. You might have to shorten your hose a bit to compensate. Just make sure you use the same diameter jetlogs that you ran on your 6 with those pumps on your eight. which would be around the 28 mm diameter.
Also try and keep your high pressure hose from the discharge of the pump to the powerjet as short as possible so as to reduce frictional losses within the hose to a minimum.