Firewire (800Mbps) is at best about 1.5 times the speed of USB2 (480Mbps).
Current cheap sata SSD's have transfer rates of round about 400 M
Bps (notice the big B, so bytes instead of bits).
With one byte being 8 bits, it means to max out the speed of a regular cheap SSD you'd need a communication speed of approximately 3200 Mbps, actually due to overhead in the Firewire communication protocoll (error checking etc) it would even need to be higher.
You clearly can see the Firewire 800 is going to be a severe limitation to the speed of your SSD.
Actual speed of data transfer will be hardly faster then your internal spinning drive (if at all), it will feel a bit snappier (just a little), the reason being there are no search time lags on an SSD, because there is no magnetical reading head which needs to physically move around to go and fetch data spread all over the magnetical disks.
Even if you build the SSD into your iMac, it will still be limited as the interface available is only SATA 2 which allows a real life speed of round about 300 MBps, still not the full speed a cheap SSD is capable of, but at least a BIG improvement over a spinning drive. I get on a cheap crucial MX500 writes of around 250 MBps and reads of 270 MBps.
Those iMacs are actually relatively easy to work on (magnetic glass plate), I'd really try to clone the drive to an SSD, open the machine up and swap out the old drive. Really not that hard to do if you carefully follow the many available (video) tutorials on the web.
There is only one step where you have to carefull, and that is when all the screws of your display are out, to very carefully flip it forward so you won't pull on the connecting cables befor you disconnect them (wrecked the connector on a back-light power module myself that way, 60 US$ replacement...). I turn the machine around and stand behind it when doing that, quite easy that way.
As well you'll need the Macs Fan Control application afterwards or you'll have a huge turbine sitting on your desk afterwards (due to the missing special Apple hard drive with built in temperature sensor)!
I actally use that piece of software anyway, I find the regular fan speed controllers let the temperatures go up too high in the iMacs...
They say these machines are too old, but with an internal SSD they still have some decent useability left.
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Again... it's not really worth trying to upgrade a 2011 iMac any more.
Either get a 2019 (very nice, but get an SSD inside), or "wait".
There might be a new iMac this year, but they seem to get released in "2-year cycles":
2015... 2017... 2019... 2021 for the next one?
Do you want to wait that long?
Kind of disagree.
An SSD upgrade is very cheap, not too hard to do, and the ssd is always re-usable after you finally dump the machine (or when the graphics card dies) , and does make an older iMac (especially the 2011 i5 or i7 ones) pretty darn useable.
