The Uni-Q driver in the Reference series and Blade/Blade 2 are the same.
In the current R series, I do think the mids and treble is a bit more laid-back than the LS50. The most direct comparison to me would be the LS50 to R100, as both are a single driver and it's the same one in both. The main differences then become the cabinet and the crossover. Look at the differences in the two plots and perhaps you can see how the LS50's deeper cabinet, improved port configuration, but primarily curved front baffle improves off-axis performance of the tweeter and affords deeper bass. The LS50 is a speaker which many people prefer to listen to slightly off-axis, and I am one. On axis, it can be a little "too truthful" for some ears.
Using the same Uni-Q driver in the R300 means it would be crossed over to the woofer sooner. The larger cabinet with the Uni-Q driver higher should mean a little more laid-back performance of the tweeter, and more pronounced mid-range into mid-bass from the large woofer. This is similar to what I find with my R500 speakers. I can be directly on-axis and not feel the same about them as the LS50. I think you'd really like the R300 for this reason.
KEF R500, R200C, R100, R50, SVS SB2000 sub, NAD T758v3 AVR, Pro-Ject RPM1 TT w/ Ortofon 2M Red, Bluesound Vault2, LG UP875 4K BlueRay, Samsung UN55JU6500F 4K LED TV, KEF M500 headphones, Sony CDP-CE500 CD changer. Desktop: NAD M10, KEF LS50, KEF Kube 10b. Guest room: Hafler 200 L-R, KEF reference Model 100 center, KEF iQ10 surrounds, M&K V-90 sub.