In simple form Range = (speed/tsfc)*(lift/drag)*natural log (Initial weight/final weight).
tsfc enjoys a curvilinear relationship with thrust. Typically there is a point on the operating characteristic where it falls to its lowest value with increasing thrust and then begins to rise.
Another way of looking at it is to say that within a family of engines and operating characteristics the law of cussedness of nature means that you can't always get what you want. You might want to trade-off a slightly worse tsfc for longer engine life for example. But if everyone involved in the whole design and buying process is in sympathy with everyone else, the harder you work the engine at its optimum altitude, the better performance you get... within reason.
http://s28.photobucket.com/albums/c2...h/tsfc_jdb.jpg
and "Gas Turbine Performance" by Walsh and Fletcher, pub Blackwell and ISBN 063206434X page 356 onwards might give further insight.