In Test Plan I've assigned priorities to test cases. Each test case has several instances in Test Lab across many test sets. Because of the number of test cases and the amount of replication, defining priority in Test Plan saves me a significant amount of work. However, now I can't modify priority on a per-test instance basis.
So for example if I create Test_Case_1 with a priority of P1 in Test Plan, create instances of Test_Case_1 in 20 different test sets in Test Lab but want 1 of those 20 instances to be P2 instead, I'm unable to do so. I would instead have to leave priority out of Test Plan and go through each of the 20 instances assigning priority individually. With thousands of test instances, that becomes a painful task.
Re: Modifying Priority per test instance in Test Lab
So, you have two levels of priority : the one you assign in the Test Plan and the one you assign in the Test Lab.
The priority in the Test Plan is the default priority in the Test Lab.
I would add a user-defined field (UDF) to test instances in the Test Lab and initialize its value from the priority found in the Test Plan when a test instance is added to a test set.
As you can add multiple tests to a test set, the more secure way to initialize the UDF would be through a database trigger on the TESTCYCL table. I'm not sure that you can initialize UDF's for test instances in the workflow.