Yes they can all be upgraded although the MK1 and early Impact-X units would need more than it is worth, money wise, to upgrade to the AMP regulator, found on the later Impact-X and MKll guns, as one would need to replace the whole trigger block as well as many parts with it. Something I did not do when upgrading my Early Impact-X. The cost to upgrade my early X gun to be mostly equal to a MKll goes something like this; ;^)
Impact-X $2,100
Aztec scope $440
FX Rings $60
580 cc bottle $350
Huma regulator $120
Regulator rod $30 Factory reg rod was bad when new
.177 barrel kit $400
.30 barrel kit $320
.25 700mm liner $110
Ergo Grip $40
STO Moderator $130
K&L Trigger guard $130
K&L safety $40
UTG Bipod $135
Mono pod $60
Valve body $35
Power Kit $40
ARCA rail $40
Wika Gauges $115
Extra mags $270
CF Barrel shroud $45
Spare parts $150
Impact & parts $5,160
Not really this mostly the addiction talking here.
You can actually get by without most of these mods.
The mods that make a big difference are the STX barrel and liner, the AMP reg, the power plenum and a bigger transfer port in your barrel and in the valve. But lets not forget that pesky little C3 bumper as well.