The Max "high pressure" system is nice for structural steel, but overkill for fastening Hardie siding to steel studs. The price for the whole system is +/- $2800.00.
ET&F makes code approved pin fastening systems for attaching building materials to steel studs and concrete. Their Model 510 tool (with a standard air compressor) is designed for fastening Hardie to steel studs. The pins come in 1-1/2", 2" and 2-1/2". The fasteners are covered in James Hardie's ICC code approval report NER-405 and their "Best Practices Manual". There is also a Florida product approval.
Trim is installed using an ET&F Model 110 tool or 210 tool. These tools and pins are designed specifically for fastening Hardie trim to metal stud framing. They also make a systems for attaching James Hardie siding direct to concrete block and plywood to metal stud framing / steel trusses.
The phone number for ET&F Fastening Systems is 800-248-2376. Their web site is http://www.etf-fastening.com