Permission caching in Tcl isn't safe because there's no comprehensive cache clearing when a permission is changed.
Also, "Checking in tcl is by far less consuming due to caching" is not, in general, a true statement. Most sites will run with permission caching turned off due to the problems caching can cause. And if a query returns (for example) 10 rows with the permission check, and 1000 rows without the permission check, doing 1000 permission checks in Tcl, one at a time, after the query, is NOT likely to be faster even IF you have caching turned on. And if you don't, doing 1000 additional queries isn't going to be fast.
Of course a lot of queries don't fit that profile, but many do, so you should be aware that your general statement about tcl checking being faster is simply wrong.
The "exists" permission check is the recommended way to do it and typically is very fast, and indeed is used throughout the toolkit. My guess is that it's blowing out the query because of the way it's being used, i.e. in a subquery.
Do what you want locally, but I don't consider moving the checking to tcl to be something we'd want committed to our CVS. Someone who uses assessment ought to work on that query, though, I'm sure it can be sped up considerably.
