You'll get better hop utilization if you boil the full amount (take into account potential losses from boiling a full 5 gallons for an hour).
Some say the large amount to small amount of grains can affect the pH of the wort. I used to do this (and still do when I do extract), and have NEVER had a poor effect on the beer (I NEVER have checked the pH on my beer/wort).
If you used to use the late water addition to cool the wort before pitching the yeast, keep in mind that you will have to cool a much larger amount of water. I suggest a wort chiller if you go this path. You'll use all the ice in your house trying to get an ice bath to do it.
Make sure your pot can take the full amount and not boilover.
Congrats on moving to a full boil!