Many are confused about the effects of the upgraded items on mechs. The tool tips are pretty weak in their explanations. So I thought I would break it down here.
The difference between Mk1 items and Mk2 items is simply effectiveness.
The difference between Mk2 and Mk3 items is simply that you can use the Mk2 variant twice in one life. The global item cool down applies between uses.
Some basic examples of differences between the item tiers:
Mk1 Detonator has a large explosion radius and deals 36 damage at the center.
Mk2 Detonator has the same radius as the Mk1 detonator and deals 55 damage at the center.
Mk3 Detonator has the damage of the Mk2 Detonator and 2 charges you can use each life.
Mk1 Repair Charge replenishes 115 HP.
Mk2 Repair Charge replenishes 170 HP.
Mk3 Repair Charge replenishes 170 HP and has 2 charges you can use each life.
Upgrading items is fairly expensive. Upgrading a Mk1 MG turret to Mk2 costs 2082 HC; same for upgrading Mk1 Repair Charge to Mk2 Repair Charge. Some item upgrades are cheaper, such as Mk1 Shield to Mk2 Shield which costs 1121 HC. Upgrading from Mk2 items to Mk3 items is even more expensive. For example, upgrading from the Mk2 Repair Charge to the Mk3 Repair charge costs 2563 HC. It is unclear whether this is adjusted by the number of people purchasing each item globally (supply and demand) or if it is set based on power of the item.
In my opinion:
Seeing as the G2 variant mech that is required to get access to Mk3 items costs quite a bit themselves, it is unlikely that anyone who purchases a G2 mech will get to immediately enjoy the benefits of upgraded Mk3 items without grinding a bit first. You will see more tangible benefits right away from upgrading internals first because they are used whenever the internal's activation conditions are met and have significantly cheaper upfront costs (a little over 1k HC cheaper on average). For example the Mk3 Deflectors reduce damage by 30%, rather than the Mk2 Deflectors 20%, every time you are boosting or dodging (excluding turret mode). Since you can use that every time the condition is met, rather than only 2 times per life, and at a cheaper upfront cost. I think it is best to focus on upgrading internals before items.
Hope this post clears some things up about the difference between items!