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Currently using 12-25 with 53-39 up front (I think). If I swap to an 11-28 cassette will I notice much difference on the climbs (ie slightly easier) or would I need a compact for that?
You'll have 3 more teeth, so yes it will be slightly easier.
Yes it helps I run 53-39 with two sets of wheels one with 11-28 and other 11-25. Most of my road training involves riding lake district passes, on the 11-28 my times are generally a good ten minutes quicker on a loop of say 4-6 passes. The 28 sprocket allows me to keep my cadance high and therefore not lose much momentum in the tough sections.
And more speed in the 11 cog?!
I wasn't sure if 3 more teeth would help that much, but i guess they will!
And more speed in the 11 cog?!
Chuff me I can't even push 53 - 13 at a reasonable cadence I would be doing 35mph plus!
Remember its lumpy to say the least round here and I only really get to use the 11 sprocket on the descents. The 28 matters if the hills are big and steep enough. You.. or should I say I 😳 begin to labour and stall on the 25 on Honister, Hardknott and the likes. Even Jebby uses a 28 when he storms the Fred Whitton in record times 😉 talented git 
on a 14% hill 25 feels like I've run out of gears, i can still ride it but its hard work. 28 should help then reading comments here.
The 28 will give a gear ~12% lower than the 25, simple maths.
but the hills will be just as big
If you don't want to go to the expense of a compact chainset, aye. Most modern mechs will cope with a 28t - check you have enough chain links in when fitting too.
It will give you even bigger gaps between ratios, but that sounds as though it might not matter if all your riding is straight up and then straight down again.
once you know how certain ratios feel etc then use this to look at ratio options:
http://www.sheldonbrown.com/gears/
current 53/39 12-25 gives GI range of 116.5 to 41.2
116.5 85.7
107.5 79.1
99.9 73.5
93.2 68.6
87.4 64.3
82.2 60.5
73.6 54.1
66.6 49.0
60.8 44.7
55.9 41.2
Using 10 speed
Change of cassette to 11-28
127.1 93.5
116.5 85.7
107.5 79.1
99.9 73.5
87.4 64.3
77.7 57.2
66.6 49.0
58.3 42.9
49.9 36.7
41.2 to 36.7 is a useful drop
I did roughly this switch last year for a trip to the Lakes - I think I may have gone from 24 to 28. It does make a noticeable difference.
Depending on how many gears you have at the back, you may start to find the spacing is a bit much. My bike was only 7-speed and the wider range is definitely a compromise.
My 105 rear mech should be ok? Ordered ultegra 11-28, however checking shimano site for comparison tells me that I need an ultegra or durace rear mech? 105 says it's good upto 28t and 11t as min.
Yeh, Ive used a 105 with a 28 before (on my mtb)
Wondering if I've been a bit hasty in my purchase, 5 weeks before event, can my fitness improve further not to need the new cassette? Training every week at mo although not on long hills.
what event are you doing?
Chiltern little dipper sportive 4 cat 1 climbs?
Wondering if I've been a bit hasty in my purchase, 5 weeks before event, can my fitness improve further not to need the new cassette? Training every week at mo although not on long hills.
better to increase cadence up hills anyway rather than grind up.
I dont know that event but you should be ok?
How steep are the hills?
As fitness improves drop back to the 12-25?