Here is the result of my conversation with Crucial. Given their quality and reliability as manufacturer and supplier, I personally am tempted to spend a little more and source the upgrade kit directly from them. If any problems then should be easy to have it fixed.
[Mark: Hi Can you tell me if CT3309352 - Crucial and CT2K4G3S1067M are the same part numbers for a Crucial upgrade 8GB kit for the MacBook Pro mid 2009 13" or are these two similar but different units produced by Crucial?
Crucial Agent : When you order by advanced search or by motherboard/system information online, you are shown the “external” part number CT3309352. The part number that you see on your module is the actual module base part number CT2K4G3S1067M. If you order by advanced search or motherboard/system information, the external part number is included in the packing list with your memory module. The modules are exactly the same. They are the same part. And will work in your MacBook Pro mid 2009 13".
Mark: Thank you. So you are saying depending on the method of search, you will get two different numbers but they are a reference to the identical unit that will be compatible, with above MacBook Pro?
Crucial Agent: For some systems yes. Some times you can look up just by the part number CT2K4G3S1067M and put it in to find it, but it is not always show any results when doing this. Normally only the servers show the actual part number. But this one is listed as compatible for your system and will work.]
So there you go this should help.
Cheers
Mark
While I am not a fan of of mixing and matching memory, Crucial, a manufacturer of quality memory components, has this on their website:
Memory is designed to be backward-compatible, so generally speaking,
you can safely add faster memory to a computer that was designed to
run slower memory. However, your system will operate at the speed of
the slowest memory module.
So, technically speaking, the answer to both your questions is, yes it will work and no it won't make a difference as you can mix and match the different speeds.
They also go on to say...
Keep in mind, that the right memory for your computer is the kind of
memory it was designed to take. Check your system manual or look up
your system in the Crucial Memory Advisor or System Scanner tools to
find the memory guaranteed to be 100 percent compatible or your money
back!
Well, the "system scanner tool" is for PC's but the rest of the statement is absolutely true. The last part about "money back" is why I only buy from reputable manufacturers like Crucial (PNY and Kingston are excellent choices as well).
That said, according to Everymac.com, the maximum supported memory configuration is 32GB1. Personally, I would (at minimum) sell the existing memory that you have (4GB modules are going for around $20USD on eBay) and instead buy a 16GB kit so you can max out your memory in your iMac either today or at a later date.
My personal iMac runs with 32GB of RAM which allows me to virtualize a Windows 10 machine, and two FreeBSD servers (one for development/testing and the other for a Plex media server). I am a firm believer in the old axiom "it't better to have and not need than to need and not have."
1 Everymac.com: Officially, this model supports four 4 GB modules -- one in each slot for a maximum of 16 GB of memory. However, third-parties have discovered that it actually will support 32 GB of memory using four 8 GB modules.
Best Answer
Apple has a support document explaining this, see https://support.apple.com/en-us/HT201191#27inch2017 for your model: