Jeff,

If all you want is to get your fly down a few feet deeper during the retrieve, or have it drift deeper in moving water, then a sink tip is okay, but...

I do a LOT of fishing in lakes...I do have a couple of sink tip lines, but find that in a lake situation, especially deeper than 10 feet, you'll need a full sinking line to fish effectively.

Even a type VI 'sink tip' won't get your fly down past ten feet for more than just a few feet of a long cast. And even that only on a super slow retrieve.

If, as you mention, you really want to fish in that 15 to 20 foot range, you'll want to buy the fatest sinking line that your rod will throw. Look at the Rio Deep 7 line, or the Rio Outbound in the type 8 for your rod weight (you'll want the 6 wt. Outbound full sinking line for your 5 wt. rod). Either of these lines will get you down right quick (you'll still need to wait about 20 seconds or so to get down to fifteen feet before you begin the retrieve), and help KEEP you down during the retrieve.

Buddy