SR4 had the option to upgrade a commlinks stats (Response, System, Firewall, and Signal) independently. While the same mechanic might work for a device in SR, to my mind it'd have to be something like upgrading data processing and firewall, and as a result of that having the mechanical device rating of the device rating increase if both stats were upgraded.
Given how expensive decks are, however, I'm not sure it's going to be possible to implement without unbalancing the decks (i.e. I think it'd have to be balanced in such a way that a decker can't just buy the cheapest deck there is and upgrade all of the stats to 6s for far less than the best deck out there). That might be a tricky proposition; somewhere between 10 and 100% of base cost * new rating, or * rating increase?
So upgrading the cheapest deck we'd be looking at 4 3 2 1 stats. To bring it to a device rating 2, upgrade the lowest stat so that all stats are two or higher. In the extremes proposed, that means 2 * 10% * 49500 = 9900 or 2 * 100 * 49500 = 99000. The first puts the total cost of the device at 59400, which is slightly more than the second cheapest deck which has 4 3 3 1 stats. If the latter suggestion was used, it'd be just 4950 for the one rating increase, which definitely seems too cheap.
Bringing an Erika MCD-1 (4 3 2 1, base cost 49,500) up to the level of a Fairlight Excalibur (9 8 7 6, base cost of 823,250) would mean raising four stats by 5, which in the first example presented above would be the equivalent of (9 * 4950) + (8 * 4950) + (6 * 4950) + (7 * 4950) = 44550 + 39600 + 34650 + 29700 = 148,500, for a total investment cost of 198,000. To me, this means that new ratings needs to be more expensive than 10% of base cost * new rating.
Using the same example, at 20% of base cost * new rating the total investment would be 301950, so closer, but not quite there.
At 30%, 495000
At 40%, 643500
At 50%, 792000
Using the same logic for, say, an Ares Alpha with base device rating 2 (meaning data processing 2 and firewall 2) and a base cost of 2650, the same logic could be applied. Upgrading to device rating 3 would mean upping DP and FW to 3, or 2 * (3 * 265|530|795|1060|1325), depending on percentage used.
At 10%, 2650 + 795 + 795 = 4240
At 20%, 2650 + 1590 + 1590 = 5830
At 30%, 2650 + 2385 + 2385 = 7420
At 40%, 2650 + 3180 + 3180 = 9010
At 50%, 2650 + 3975 + 3975 = 10600
At first, 10 grand might seem like a lot for a measly Ares Alpha, but this puts it in line with the cost of upgrading the most basic deck in the world to a state of the art supercomputer. You'd essentially have a DR6 rifle, which is pretty unheard of.
Based on my math above, I'd allow upgrading Attack, Data Processing, Firewall, and Sleaze rating (leading to an increase in device rating based on the lowest common denominator) for cyberdecks and Data Processing and Firewall for all other devices (with a similar DR increase) using the 50% of base cost * new rating system.