In software terms, that's more difficult than adding support for new HTML extensions given java byte code for a web browser.
No one makes modifications that could affect instruction timings (which is a very fundamental aspect of a core), and I very much doubt such modifications are even allowed in the license you buy the cores under, even with a separate architectural license (which is for ground-up designs). Even NVIDIA who bragged about their A9 and A15 core revisions only gave feedback to ARM and had to have them actually implement the changes, and none of those changes affected instruction timing.
No one makes modifications that could affect instruction timings (which is a very fundamental aspect of a core), and I very much doubt such modifications are even allowed in the license you buy the cores under, even with a separate architectural license (which is for ground-up designs). Even NVIDIA who bragged about their A9 and A15 core revisions only gave feedback to ARM and had to have them actually implement the changes, and none of those changes affected instruction timing.