If you are happy with what you have don't fix it.
I've spent hundreds of hours since SP1 came out installing, reinstalling, testing, fixing, and generally struggling with TS12 on two systems (one a brand new "gaming" computer). I just finished patching up to 58414 on two installations on the new system (about 48 hours) and did some comparisions. On all installations of TS12SP1 58414 on both systems I found problems with missing/invisible railcars that worked fine in 49922, missing trees that worked fine in 49922, increased stuttering not to mention all the older content that was marked as faulty. (I tested by looking at the TS12 built in routes.)
Bottom line IMO; If you have a kick butt top end computer upgrade by all means, assuming you have the time and bandwidth to work through all the rebuilding and fixing. If you have an average PC that is performing well with 49922 why bother? I have not seen any compelling reason to update other than the merge fix. This Service Pack did not fix much and did introduce several new problems. It is much more of a marketing patch rather than a performance/repair patch IMO.
As my Daddy used to say, "If it ain't broke don't fix it."
Edit: HF3 did improve things substantially but did not bring performance of SP1 up to the level of 49922 on my systems.