Is exceeding 40T derailleur capacity to 41T ok

A 3500 Sora 9 speed medium cage (GS) derailleur should be fine, they can handle up to 32t or 34t cassette used with a triple chainset. Shimano's rear derailleurs seem very similar in use, I don't think you get any better shifting with 105, it's more down to the cables and STI.

I ran 11-30 (and 12-27) with Tiagra 4500 then Sora 3500 medium cage derailleurs and a triple chainset. I didn't like the gap due to the absence of 13t on the 11-30 so fitted the 12 and 13t from a hardly used 12-27 I picked up on ebay.

Am currently using SRAM PG-950 11-28 (11,12,13,15,17,19,21,24,28) with compact chainset. You may be able to find Shimano HG61 in 11-28 (11,12,13,14,16,18,21,24,28) if you prefer that set of ratios.

It'll be fine.  Exactly the setup my wife is running on her Equilibrium. 

Yes, 10 speed is compatible with 9 speed. The pull ratios changed when they went from 10 to 11 speed.

What chainset are you using for this?

A 110bcd chainset can only go to 33t inner chainring.

I'm looking to have something like a 48/30 chainset, as the 30 is great for spinning up alpine climbs and 48 would be ok for the descents.

I'm not talking about old square taper ones, but newer hollowtech

It will work just fine. Go for it. As stated, manufacturers always err on the side of caution. I have successfully run a 30 tooth sprocket on an ancient Dura Ace derailleur which specified a max sprocket of 25 (and with a 52/39 front).

I've run an 11-32 with a 52-39-30 using a RD-5701-GS (later 105 mech but remember its the shifter that does the indexing) The box states that it is up to 30 with a triple or 32 with a double but it works perfectly on mine with 32 on a triple. Did all 100 Greatest Climbs like that so should be OK

That's a very big range. I would suggest if you really need a 30 tooth large sprocket you could probably look at downsizing the 52 chairing to a 50. This would be cheaper than a longer much, but would mean moving the front mech down and so recabling and tuning.

11-28 seems fine with a 30 tooth small ring.

Is the chainset a triple? Seems like a big jump otherwise, if it is a triple the chance of ending up in small small or big big seems more remote. Agree with the statement above make sure the chain is long enough for big big (fit round the ring and cassette with no mech then add 2 links) then see what that means for small small it may or may not result in the chain passing from chainset to mech rubbing against the chain passing round the top jockey wheel.

Not desirable but not catastrophic and would only happen with using the gears on the wrong combination.

Because there are differences in mech position, chain line and chainstay length between bikes, manufacturers are always going to give themselves a bit of a margin. mechs that claim a 28T or 30T max sprocket can usually cope with sprockets a couple of teeth bigger, and i doubt you'll have any issues with a capacity that's one larger than what's stated.

Oh, and if many miles have been covered, the newness of chain, cassette and mechs will probably make more difference to shifting that the bigger jumps across the cassette.

Oh, and you could also mash up your own cassette, like this: http://harriscyclery.net/product/harris-custom-century-special-13-30-9-s...

Shifting might not be quite as slick, though, as the ramps may not line up as well. (not 100% sure about this).

See this page for Sheldon Brown's comments on exceeding capacity:

https://www.sheldonbrown.com/harris/derailers-rear.html#capacity

See the links in the text to understand why he says that.

So, easy questions first:

Downsides to moving to a wider range cassette:

• bigger steps between gears mean you need to be able to ride with a slightly broader range of cadences.
• slightly less quick/smooth shifting, but it will still be perfectly acceptable unless you're really fussy.

I believe the 105 5701 rear mech has the same cable pull ratio as the 9-speed shifters, but you may want to double check to ensure compatibility. Some recent road mechs (eg 4700) have a different pull ratio so watch out.

Now, as for capacity, you may be able to get away with it. My advice would be to make the chain long enough that cross-chaining in the big-big combination doesn't rip the rear mech off! If the chain goes slack in the opposite, small-small combination, this is less problematic IMHO. If you are confident that you wife can avoid using the small-small combo you should be able to get away with it. You might even find, depending on exact chainstay length, that it actually works fine in both extremes. (If I'm thinking straight, the mech needs extra capacity to deal with slightly longer than optimal or slightly shorter than optimal chains because chain length can only be set in 1" increments - you might get lucky and have the ideal chainstay length for your sprocket sizes)

You might be better off, however, fitting an mtb 9 speed rear mech with more capacity eg RD-M591 or 592, which both have a capacity of 45t I believe. It will be compatible. You can also go for an even lower low gear, should you wish - you can get 12-36 that would work with the M592. Don't buy an MTB 10 or 11-speed mech however, as their pull ratio is incompatible!