Am I right in saying I don't need to worry about enabling trim as the 840 pro has a built in garbage collector.
TRIM and garbage collection are different things, although they both help work toward the same goal of never running out of free NAND pages. In most cases they work well together. The TRIM command simply informs the SSD that there are logical blocks that have been deleted in the OS. Then the SSD firmware can clean the NAND pages associated with those logical blocks.
TRIM has some other benefits too, such as a reduction in write amplification.
With garbage collection (and no TRIM), the only way the SSD finds out about deleted logical blocks, is when the OS decides to write to those same logical blocks again. This happens more often as the drive gets full. If the OS asks to write to more blocks than there are free NAND pages, then the performance suffers.
If I was going to choose a drive to run without TRIM, I'd pick one with more generous over-provisioning than the Samsung 840 Pro.
http://www.anandtech.com/show/6489/playing-with-op
The 840 Pro has about 7% NAND capacity set aside as spare area, while others such as Crucial M500 have 14.5%.
http://www.anandtech.com/show/6614/microncrucial-announces-m500-ssd-line-of-ssds
That is not to say the Samsung 840 Pro is a bad drive - it's a very good one, but it wouldn't be my first choice to run in a no-TRIM environment.
Everything has garbage collection now. TRIM may cause a performance hit depending on the firmware and controller though, but nothing too bad. In general, it's best to turn it on.
Some people have had problems with Groth's TRIM enabler recently, myself included, although I'm running a Corsair Neutron GTX so it might just be a wonky drive. I forced a TRIM in single-user mode and I'm fine now though.
True, every modern SSD has garbage collection. TRIM should improve the performance over time in most cases, but it seems that some drives using Sandforce controllers have problems with TRIM and for those, it might be better not to use it.
http://www.anandtech.com/show/6107/corsair-force-series-gs-240gb-review
However I believe the Corsair Neutron GTX uses the LAMD controller, so that shouldn't be a problem in your case.