The Tarmac came with a 11-30 in the back. If I do change to 52/39, then I'll probably have to change the rear to give me a 32 or 34 cog. However I haven't looked to see what an Ultegra Di2 GS cage DR will handle, nor have I compared ratios and gain yet. On my Paramount after I've got my legs back from my winter of laziness, there are times I feel a 53/39 with it's 11-32 would be ever so slightly nice at times when I have a longer downhill segment. But only ever so slightly.........

I also see more new bikes going away from the 50/34 in favor of the 52/36. Or at least from the bikes I considered purchasing recently, they all had 52/36 in the front. With 11-30 on the back, it lets me work the bike as a 1x and I only shift to the small ring when I need a lower combo than the 52/30. Which isn't often for the short hills here unless I'm worn out from a long ride.

Typically I'm 85 - 90ish RPM. Spin out.... that's a hard one to answer, while I can briefly maintain about 120 to 125 maybe better..... briefly, that's usually in the 13, 14 or more cog when I'm accelerating. By the time I get to the 12 and 11 cog I've only got enough oomph to do maybe 100 RPM or so. But yes I do get up to 38 or better mph on some downhills. But none of the long hills I ride here are smooth enough to allow pucker factor to not overwhelm me and limit my speed.

I also like a short crank. 165 mm on all my bikes. Even though my inseam is 34.5" from ground to snug in the crotch. I once tried a 170 or 172 crank on my Paramount for 3 maybe 4 months and didn't like it. It was a Shimano 105 5800. I found that chain wheels were almost as expensive as an entire crank set too. I also had trouble finding a 105 5800 in silver at that time in a 52/36, so I bought a 50/34 that had the 165 crank, swapped the rings and sold 50/34 with the longer cranks on ebay.

The Tarmac came with 172.5 Ultegra crank set. I had a 105 5800 with 165 mm cranks I was intending for an group set upgrade on my Raleigh that I've never gotten around to doing. Instead had them put it on the Tarmac when they built it.

The 58 cm Tarmac I test rode had a 175 mm crank. I rode it for 10 miles and it wore me out with all the thrashing I felt like I was doing. Immediately after that I rode the 56 cm frame Tarmac with the 172.5 cranks and I felt much better and wasn't nearly as worn out after the same 10 miles even though my times and stats were better.