Additional Connections

If I were to add an addition connection at the firewall so I could unplug the engine bay side of the harness and leave the cockpit side intact. Would that change anything?

It would add a bunch more terminations, a couple of boots and usually some expensive connectors When counting circuits to determine the number of pins required, don't forget to include pins for shields.

It usually make the ECU to bulkhead harness pretty easy since many of the splices can occur on the engine-side of the firewall.

Technically, it won't be going through a firewall. I am building a dragster and there is a "blow Sheild" behind the seat. I want to be able to unplug the main trunk of the harness that goes past that in order to be able to service and clean since it will be exposed to oil, rubber, dirt, etc.

I have included a picture of what I am referring to.

I build harness for Formula cars, and we often use in-line connectors for sub harnesses - say a rear-harness connection near the engine/gearbox junction, so when the gearbox is removed, the wiring and all the parts attached to it can go together.

Thoughts.

As David said in the first response, it's mostly more work and expense. However, it does add a lot of convenience and can be a big time saver.

It doesn't technically need to go through a firewall or bulkhead. You can weld a mounting tab to the chassis rail, to bolt the 'bulkhead' fitting to, and run the permanent wiring to it's fixed side, and the removeable wiring harness can be connected to this fixed point. Some may leave both ends loose, especially sub-harnesses, but while fixed points add a little weight and work, it can beat the heck out of trapping and damaging wiring, or trying to locate the other end in the dark, rain, and/or in a hurry.