cyclezen

As @msu2001la noted, the RD cage does have limits... mine being 6600 short cage , I've had no issues with a 27 cog, works great. I've not used 28 or larger because those cassettes don't have the cog range I prefer in the 'meat' range - single step 14-19... I'll often use a wheelset I have with a 13-27 for days when climbing is long, intense, or predominant... otherwise 12-25
as for compatibility of RD - the Brifters/shifters determine compatibility... they decide between 10 spd and 11 spd... Ultegra 6600 or 6700 are interchangeable for 10 spd, 6800 & 8000 should work - although I've not have personal experience with that yet... I've used the older 9 spd 6500 RD on a 10 spd setup for 5 months (2.5K miles approx) and that worked very well.
as long as the cog range isn;t too large or too broad a range for the cage style.
Cage length will determine max cassette range...
I've NOT used any 11 - ? cassettes - 11 tooth cogs are a joke for me ... would/could never use... I can spin a 12 (on a downhill) up to 100 rpm if needed - that's just over 50 mph... LOL!
at 45 mph, I relax and enjoy the ride !!!
I have, just a month back, replaced the 53/39 std ultegra crank with a NEW Ultegra 52-36 Mid-Compact crankset (which is '11 spd') works great !!! Awesome improvement as I continue to approach the grave... LOL! 11 spd cranks, RD and FD will work on 10 spd as well....
Your options for replacement/upgrade are very broad - without having to do full gruppos... the equivalent Dura Ace stuff (or lower 105) also works. Britfers determine the 10 - 11 spd world.
Ride On
Yuri
EDIT: I do have a wheelset using Conti 700x28 tires (n HED 23mm wheels) and have plenty of room/clearance on the stays, should I get a wooble or broken spoke issue during a ride... I'm eyeballing 3mm space on each side..? normally I ride 25c tires with no issues...