The answer is Yes and No, depending on the app developers. I purchased iPhone 6 Plus for the hope that I can see bigger text. But apparently, not every user or developer thinks the same.
On my iPhone 6 Plus, I turned on the "Zoom" mode in "Display & Brightness" for my aging eyesight. What it does is to render the text bigger. It works very well in Apple's own apps. The fonts are large and crystal clear. But when it comes to third-party apps, it is a mixed bag. There are some apps that are optimized for 6/6+ that render fonts beautifully and in large-enough sizes. But there are a majority of others (alas) that claim to be 6/6+ optimized but effectively make the rendered font almost 50% smaller than before the update. For example, 1Password, Flipboard and Pocket rendered fonts much smaller after the update. I regretted the updates and have contacted the app developers. On the other hand, the iCab Mobile browser rendered fonts as it should, comparable to Apple's own apps.
Now when I see the app developers release any 6/6+ optimized updates, I always take it with a grain of salt. A lot of them just change the resolution settings, put in higher-resolution graphics and call it an optimization. What they don't realize is that many people who buy these larger iPhones are expecting to use the bigger screen real estate to see bigger fonts, and the app developers need to tweak the app layout and design to achieve that. Some of the third-party so-called "optimized" apps display a lot of white spaces in their app now that the screen is bigger and their app is still using small fonts. For me, that is just stupid. Why I am buying a premium phone with a gigantic screen just to stare at white spaces? The app developers need to utilize that space for some useful purposes. I know some people would argue that they want to see more content and they prefer small fonts, but for my eyesight, a bigger font is not just a preference, it is a requirement. At least I want my apps to give me the option to increase font sizes. Or at the very least, these apps need to honor the global font settings that I made on my iPhone.