Check out this link in regards to Winchester 571. Sounds like HS7 is nearly the same so if you didn't like how dirty the HS7 burned then the W571 may not impress you much either.
Sounds like we have the same goal...which is trying to get these hunting 200gr XTP to break slightly over 1200 fps. The N-105 was probably the best we can get over the counter at stores. The only way we can probably get better powder was if it was in the same burn speed but sprayed or coated with a Nitroglycerol coating.
See info below:
Adding nitroglycerol to the traditional single base powder makes possible in addition to geometry and coating a third controlled variable of ballistic properties: energy content. Vihtavuori calls powders that have nitroglycerol added (maximum 25%) high energy NC-powders, which form N500 series. Geometrically the powders in the N500 series are equal to the N100 series. Although these new powders have a higher energy content, they do not cause greater wear to the gun. This is because the surface of the powder has been treated with an agent designed to reduce barrel wear. N500 series powders work well at different temperatures, even better than the traditional N100 and N300 series. Temperature sensitivity naturally depends very much on the weapon and on the cartridge. The manufacturing technique employed permits a very high bulk density, which in turn makes it possible to use a bigger charge in a certain limited loading volume. Vihtavuori High Energy powders are available in three burning rates:
Burning rate like N140. Especially for the .308 Winchester.
Burning rate like N150. Especially for the .308 Winchester and .30-06 Springfield.
Burning rate like N160. Especially for .270 Winchester and 6.5 x 55 Swedish Mauser.
N550 is the best powder I can find for my rifle. All we need is this process applied to some pistol powder in the burn rates that we use.