I am currently upgrading my bike from a 9 speed to an 11 speed system. I am currently using shimano 105 components and will continue to use them within the new system. Current bike has a shimano 105 rear derailleur but does not specify if it is unique to a 9 speed system. Will I need to buy a new rear derailleur?
This page has the best answer I've found so far
Reading between the lines, the cable pulls from 9spd to 11spd are probably close enough, but the derailleur ratios are significantly different. 1.4 vs 1.7. This means that the 11 spd indexing in your new shifters will not work correctly with your old derailleur.
You'll need to change the chainwheels and the cranks, the rear derailleur, the brake-shift levers, the cassette and the chain of course. The rear wheel probably has an older hub but then if you're lucky and the hub is designed for 10-speed as well it will also take 11. The brakes won't need changing but then the newer ones are designed for less friction in moving parts. Technology trickle-down from Ultegra.
To specifically answer your question, you will have to change the rear mech (derailleur). If for nothing other than 11sp chain will not fit onto the jockey wheels correctly. You could change the jockey wheels, but they are thinner on 11sp vs 9sp.
To take this a step further, you will need to replace all of the groupset components, including the brakes as 5800 has a slightly different action/cable pull. Having tried this before with customers bikes, it causes major issues and very poor brake performance (not somewhere you want poor performance).
On a positive note, 5800 brakes are leaps and bounds better than their predecessors!
You won't need to replace your rear derailleur. Cable pull is determined by the shifters so if you're getting new 11 speed shifters you can use ANY rear mech as long as it has a long enough cage to accommodate the largest rear sprocket. For example i'm pretty sure that if you have a short cage rear mech you won't be able to run an 11-32 cassette. The rear mech has an infinite range of movement between the high and low stop screws so as long as they are adjusted properly you'll be fine. B-tension screw is what brings the cage in or out to accommodate the largest cog.
The reason for any problems in compatibility is that for 11 speed there are more cogs (11) packed into the space between the hub flange and the frame dropout so the spacing between cogs is narrower which means the chains need to be narrower and the shifters pull less cable with each click. But if you try and run a 9 speed chain and chainset the teeth are wider so the chain won't sit properly on the chainrings.
So in short you'll need a new chain and cassette and shifters but you can keep your front and rear mechs.