Looking at upgrading things on my bike and looking at my cranks, would it be noticable going from the tiagra to ultegra cranks?
I can't comment with direct knowledge but my road bike has ultegra Di2 chain set and my mountain bike has shimano XT mechanical. The difference in shifting is negligible. For performance, I assume speed increase, you won't notice a difference worth bothering.
Also depends very much on the rest of the bike. What are your objectives?
Just your cranks? Absolutely no noticeable difference whatsoever other than aesthetics.
I can't imagine a less cost-effective upgrade!
Wheels, derailleurs etc. maybe, but the cranks alone? Unless you throw in the rest of the drive-train I can't see the point.
I ran a Sram Rival compact for a while, then switched to 4500 that I had spare as an interim and then went to 6700.
The Rival chainset is probably the stiffest out of them all, but thinking back I don't remember any direct stiffness difference between 4500 and 6700 though I think there might be, just because the 4500 is a chainset I remember as not being too stiff overall.
It's probably going to be stiffer and definitely give better shifting - if your Tiagra chainset is wearing out and you can get the 6307 for a good price, then go for it. It not, save some money and buy a pair of lighter wheels - that's where you will notice the most immediate difference.
Had tiagra double on my tarmac, got a killer deal on 'shop soiled' 6700 crank put it on same bike. Noticeable difference in crank and chainring stiffness when hammering in the big ring, especially when standing up, and doing bad shifts from smaller ring to larger ring.
Also have 105 (same forging as tiagra) on other bike and the difference with 6700 is noticeable.
Shimano developed the 'hollow form' crank set used on ultegra and dura ace and corresponding large ring specifically for Di2 to provide a stiffer large chainring as the electrically shifted Di2 front mech produces a lot of torque and this was bending the standard ring found on 105 and Tiagra, causing shifting issues.
Whether its worth the money for you? Not unless your current rings are worn out...
I can honestly say that shifting in the front ring between Tiagra, 105, R565 (or whatever it was) and Ultegra on all the bikes I've owned would be impossible to tell the difference between in a blind test. My Tiagra shifts just as well as my Ultegra other than lever throw but that's not the fault of the FD or the chainring. I also dare anyone to pick up on a difference in 'stiffness' under the same conditions. The only difference is a tiny saving on weight and you'd need a set of scales to even notice that, anything else is either groupset snobbery, purchase justification or the experience of somebody so incredibly finely tuned that they can relate completely to the story of the Princess and the pea.
The only chainset I have ever owned that flexes is an FSA tempo square taper and I'm over 80kgs and I'm not shy on the watts.
I swapped a Tiagra 4503 triple for an Ultegra 6600 double, and saved about 400g in weight. The Ultegra felt sprightlier, and you do notice the difference between machined chain rings and pressed chain rings, whatever anyone says. So if you buy a good Ultegra crank second hand and sell the old Tiagra triple to someone building a tourer, the changeover can be relatively painless in my experience, and the improvement noticeable.
I have been told, by someone who knows, that 4500 cranks are actually stiffer because they are solid, not hollow.
4500 chainrings are the same as 105 (5600) but the 6700 series are hollow; Shimano claim this makes them stiffer as well as lighter, but then they wouldn't want to claim anything else. They don't say whether the hollow ones wear out more quickly.
Purchase justification crops up a lot on forums but if a new chainset genuinely makes you happy then go ahead. But it won't make you any faster.
I've upgraded my CAAD9 from a tiagra to a full on Ultegra 6800 set and the difference is astounding. Apart from being lighter, the bike handles better and braking is now not a bum twitching experience.