I have not tried it in LR, How do you go about doing it? At which point?
Just before rendering (using the
Export dialog). You can set a higher than native resolution. Same in ACR. I did not see a compelling difference doing this or upsizing in Photoshop or using Step Interpolation there but there were tiny differences (and improvements) upsizing from the raw data and very important, with proper capture sharpening. That produced the biggest differences in the tests I did.
I found Perfect Resize to be VERY slow in comparison with no benefit I could see on the print compared to using tools I already own. That said, I think Bart is spot on that the settings in these products will make a big difference IF you want to go through the trouble of wading through what can be a vast amount of options. I didn't go down that route as far as I could. I just didn't see anything compelling on the print that convinced me it was worth the effort. And I'm sure image content, output device (even the paper) would play a role here. Forget viewing distance, the differences I saw needed a loupe to see and considering the time and cost for the 3rd party products, I just decided it wasn't worth it (for me).
That said, these products may be useful in some cases. For example, Mac Holbert, formally of Nash Editions who's opinion I greatly respect told me he found these products very useful when he had to take a really tiny file and blow it up massively. In my tests, I think I only sized the 5DMII images 300% or 400%. For that kind of work, the tests I did convinced me it wasn't worth the extra effort but of course, everyone should run their own test and most importantly NOT make decisions about the quality based on what they see on-screen but rather to an output device.