A new tool, released just hours ago, aims to benchmark the performance of Second Life servers by running resource intensive scripts within a region, allowing residents to compare performance statistics before shelling out hard earnede Linden dollars on a new home. Mystical Cookie, creator of Second Life's favorite swiss army knife utility, the MystiTool, today released her "benchmark sim tester", that once rezzed, will perform a number of tests upon the island it inhabits, producing a score that can be comared against other islands.

I have to say, this was a great tool. A real winner. and worth several tims th e3 price of $L326 it's selling for at Blumfield.
I picked up one of these and I've been running tests on my sim as well as the neighbor sims to keep an eye on our development/overdevelopment progress.
I haven't checked... can I leave it up and running despite my being offline so it can collect stats over a long period of time?
I've no idea on that one im afraid CM, perhaps IM Mysti....
This tool will not log performance over time. It is purely a benchmarking tool.
I suggest you look into getting Bosozoku Kato's Scrubbie v7 for long term performance monitoring.