I installed Bennett Marine hydraulic tabs on an 18.5' Boston Whaler. It was easily the best accessory I added to the boat. With tabs, the boat would maintain plane +/-4 to 5 mph lower than without tabs. In addition, it would get on plane much faster and it was noticeably more comfortable in rough water (because we could lower speed and trim the bow down w/o losing plane).
We didn't wakeboard much behind that boat, but we did waterski some. The tabs were useful (and noticeable) for reducing the size of the wake at skiing speeds (for us, typically 28 to 36 mph) and for balancing the wake side-to-side to compensate for unequal weight distribution in the boat. I don't know how tabs would impact the wake at wakeboarding speeds.
In any event, reducing planing speed by 9-10 mph is likely wishful thinking. It seems odd that a 19' boat is having trouble maintaining plane at 24 mph. You might want to check your speedo calibration. Also (at the risk of stating the obvious), your outdrive should be trimmed all the way down if you wish to minimize time-to-plane and planing-speed.
If you do get tabs, I would highly recommend tabs that are adjustable while moving. When I installed tabs (10 years ago), Bennett was clearly the industry leader in terms of quality and engineering. Based on my experience with Bennett's customer service and the quality of the product, that's what I would use if I were doing it again. I would also recommend the helm-mounted tab indicators. I installed them after I used the tabs a bit because I found that I wanted to know if the tabs were up/down/equal at any given moment w/o having to play with them to try and guess their position. One last note: make sure you mount the hydraulic pump in a dry location. It is easily damaged by water.