We have evolved our grouping code to frequently be able to group cross-browser errors together, even when the message differs and the stack looks subtly different too. It may be that you're using an older grouping algo at our end and we could upgrade you.
The reason we don't just automatically do this is because it generates new hashes for all new errors. This means old permanently ignored errors will then come back as new active errors which would confuse people. If it's not a biggy for you, and you're keen for us to upgrade you - ping me a note firstname.lastname@example.org. I'd appreciate your feedback on it too (it's also smart enough to know that IE localises messages and will group them, defaulting to English as the message).
Regarding the overall idea of setting a custom grouping value, we would look to bring that to all providers, Raygun4JS included.