I have a 39/53 in the front and 12-34 in the back on my road bike. I used to live in a somewhat hilly area in Northern Georgia, and I sometimes found myself reaching for a lower gear than the 39 front / 34 back provided; If I lived in the Appalachians or the Rockies, then I would definitely want lower gears. I seldom used the 53/12 combo when I was in Northern Georgia, but when I did, I went so fast that I didn’t think it was safe. Smaller chainrings would have been more ideal for me, but 38 is the smallest available chainring for the 130 BCD crankset that I have, so I never changed them. I now live in a relatively flat area (Eastern Massachusetts), and I never use the 39/34 or the 53/12, so I am wasting two of my gears, and I am considering changing to a 14-30 cassette if I can figure out how to make one in that odd pattern.
I think that getting the gearing that makes you comfortable is a good idea. It will get you out on the bike more often, and risking the health of your knees or over exerting yourself is not worth it. I hear that spinning is more efficient than pushing anyway.
The 12-27 cassette is a nine-speed cassette. You want to get an eight-speed cassette to match your eight-speed shifters. You could get a 12-28 XTR M900 cassette and use the same rear derailleur you have now, but this will only give you only one lower gear. If that is not low enough for you, then you could get a mountain bike rear derailleur and a 12-32 XTR M900 cassette; if you don’t want to spend a fortune make sure that you get M900 instead of the M950; the M950 cassettes are titanium and cost a lot more. A 34 teeth cog is available; to get this, buy an 11-30 8-speed cassette (LX or XT, perhaps others?), a first position 13 teeth cog, and a 34 teeth cog; have the bike shop remove the 11 and 13 teeth cogs from the 11-30 cassette, and then have them put a 13 teeth first position cog in front and a 34 teeth cog in back; they should be able to figure this out. Depending on how your rear derailleur hangs on your bike, you might be able to get your existing Tiagra rear derailleur to work with a 30-tooth cog; your bike shop should be able to make a 14-30 cassette for you, and they could recycle your spacers and some of your cogs.
You could also change your chainrings. Your current top gear is very high. The 52/12 gear is 117 inches. You probably don’t need this high a gear, so you might consider ditching it. The smallest inner chainring that you can get is one with 24 teeth, and the smallest middle ring that you can get has 38 teeth. To avoid going outside the limits of your front derailleur, you could get 26-38-48 chainrings. Make sure to get ramped and pinned chainrings for the middle and outer position as this will help your shifting. I see from Specialized’s website that your bike has a Shimano Deore front derailleur, so going to smaller rings shouldn’t hurt front shifting performance. If you only change the chainrings, and keep the 12-25 cassette, then you will have a 28-inch low gear which is a little lower than the 32-inch gear you have now. If you go to 24-38-48, then your lowest gear would be 26 inches, but this may be too wide a range for the front derailleur to provide good shifting; perhaps someone here has tried it and will let you know how it worked out.
Replacing your crankset might be more cost effective than changing your chainrings. You can probably find a whole crankset for the price of three individual chainrings, and you could get some of your money back by selling your Tiagra crankset. A 110/74 BCD crankset should do the trick. You could get one with 24-34-46 chainrings. Perhaps you could find one that uses the same bottom bracket as the Tiagra triple crankset to avoid the cost of a bottom bracket?
If you choose to keep your Tiagra rear derailleur, change the cassette to 11-28 (these are from the nineties and should still be available for cheap: SRAM, XT, LX, or cheaper Shimano), and change the crankset to one with 24-34-46 chainrings, then this will give you plenty of gearing choices and a wide range too. This will give a low gear of 23 inches, and a high gear of 112 inches. That should be plenty.
