Stephen,
I own both the 1611 and the 1510-3 because they are my favorite 15' for a 10 line and 16' for an 11 line rods.
The 1611 is a pretty stiff and powerful rod which casts any of the short (RIO Wndcutter 10/11/12) or mid-belly 10/11 lines with ease. If you like a little deeper bend in the rod when casting without putting a lot of power into your cast, the 11/12 (or 11/12/13 Windcutter) lines will do this. I use a RIO GrandSpey 9/10 line on it and have no problem casting 10 wt sink tips of any sink rate or flies of any size with it.
Prior to the availability of the GrandSpey lines, I used a 10/11 MidSpey with the 9/10/11 RIO Upgrade added to it because I wanted a longer belly line. I found the 10/11/12 Upgrade overloaded the rod for me and felt heavy, which is why I went to the 9/10/11 Upgrade. The combination of 10/11 MidSpey with the 9/10/11 Upgrade gave me a belly length of 79 ft and it cast all of my 11 wt sink tips with ease. I never liked casting short-belly lines on it because on a 16' rod, the short-belly lines felt like I was using a shooting head, which resulted in having to use a different casting stroke and double spey technique than with the longer lines to avoid pulling the anchor.
I have used a 10/11 GrandSpey on it (did so for several months); but I don't like the heavy feel the rod has when casting it; hence I tried the 9/10 GrandSpey and really like the way the rod feels with it. I also found a DT 11 works very well on it too.
With the proper line and technique, the 1611 will cast as far as you will ever won't to cast when fishing. Casts of 120 to 130 ft are within the relm of actual fishing conditions with or without sink tips of up the type 8's. It also will cast flies of any size you may wish to fish with and will easily cast flies #6/0, if you so desire. The 1611 is the rod I use from mid-November until April 20th for chum salmon and winter steelhead because it gives such a command of the water.
My 1510-3 (which is marginally stiffer or faster than the 1510-5, unless most people cast them side-by-side they would not notice the difference) is the rod I use on medium to large rivers for summer steelhead, although I do use my 1611 on the Thompson for its summer runs. It casts any of the 9/10 short (for the Windcutter use the 9/10/11) to mid-belly lines well with any 10 wt sink tip of any sink rate you desire. I use an 8/9 GrandSpey on mine because I like casting and fishing the ling-belly lines. This combination allows me to fish out to 100 ft without the need to strip line between casts when using the floating tip and out to 90 ft without stripping line with sink tips.
The 1510-3 (or 1510-5) is a pretty stiff and powerful 15' 10wt rod. It works very well with flies from #1/0 down to as small as you wish to fish (I use mine down to #10 low-water flies) with ease. It has the power to easily cast wind resistent deer hair skaters out to 90+ ft, although I almost always fish skaters at 85 ft or less.
Both the 1611 and the 1510 have a pretty low swing weight, even though they are not really light weight rods. If you would prefer a stiff and powerful 15 ft rod, the T&T 1511 casts like and feels like a slightly shorter version of the 1611. And the 1510-3 (or 1510-5), 1511, and 1611 have that same sweet casting action and feel of the T&T 1409, they are just longer and a bit more powerful. I only wish T&T would make a 17' for a 12 line (1712 I suppose it could be called) with the same casting characteristics as the 1611 and 1510.