They are really good units - not designed to replace the ASI but instead add a backup ASI (and ALT) all together on the EFIS screen.
I suspect that the speed discrepancy reported above is more likely to be the original ASI fault rather than the G5.
Weight saving - removing the entire vac system with pump, hoses, regulator, both AI & DI, vac gauge & filters, and installing 2 G5 with backup batteries , magnetometer & nav interface is around 9 lbs.
They are approved as primary AI & DI replacements and can add HSI capability to compatible navigators (GPS, VOR & ILS)
Costs - standard EFIS kit £1725, HSI kit (inc magnetometer & nav interface) £2300, labour around £1600 depending on exact interfaces/aircraft etc, and need around £200 of other installation bits (circuit breakers, pitot/static fittings etc). All plus VAT.
Hope this helps - feel free to PM any direct questions.