Drawing down the lake for a herbicide treatment would likely be counterproductive - but depends greatly upon which herbicide will be used and your overall game plan.

Hydrilla tubers and turions can survive on/in soils for quite awhile. So, if you draw down the lake, then treat the water - the plants in the water may be controlled. However, the tubers and turions above the water line will likely begin to grow unimpeded as soon as the pool level has been re-established.

It might be a consideration to draw down, then treat with SONAR, while also adding 3-4 grass carp per pool-level surface-acre. Let the SONAR control the hydrilla that remains in the water (less product would be required in the reduced volume of water). Then, the grass carp might be able to keep the pool-level perimeter growth in check after the water-level has been elevated.

You can alway add more grass carp - if deemed appropriate. But, if you start out with too many, it is quite a challege to get them thinned out.