All small turbines will always use a lot of fuel. Even supposedly modern engines have similar SFC to the old 250 series. Problem is in the tip clearance of such a small engine relative to its size. Yes you could improve it but at what cost and for how long?
As the engines get bigger they have a better SFC but it tends to be reflected in the price. Check out a GE T700/CT7 SFC.
Actually the issue is the height of the blade passage as a function of compressor pressure ratio and size. As the engine pressure ratio gets higher, the height of the blade passage gets smaller in the final stage of compression. This limits the amount of pressure ratio you can obtain. Big engines get low SFC from big pressure ratios. Small engines, if you limit the pressure ratio to 8 or 9 are going to have an SFC of about .52 lbs/hp hr no matter what you do. You could, theoretically, add an additional spool and bring the diameter of the compressor down and increase the blade height. Problem is that you have to put a shaft through the second compressor and that actually limits how small you can make a multi-spool engine.
There have been two reseach engines that I know of that made around 750 hp and had an SFC at full power of under .45 lbs/hp hr (which is actually as good as the T700). The technology is out there to do it, and one of those engines was a low cost solution. Do some reasearch on the Army SHFE program.
All it takes is money to finish the development and certify the engines, but the technology is there.