I remember reading somewhere that you could but you would have to adjust your hops. Most additions in extract/partial boils are tuned for 2.5 - 3 gallon boils expecting 2+ gallons of water to be added afterwards.
Try running recipe through a recipe builder, set it up originally as your normal partial boil, then change to 6.5 gal boil and watch how your numbers change.
I start with a 5.5 gal boil and have always had great results. You can even use a lid to control boil off, but you have to watch it closely so you don't get a boil over!
The benefits of home brewing is that you can experiment with both and see if there is a difference in the end product.
Full wort boils (with no top off water) "generally" make better beers, especially in the case of hop forward and/or paler-colored beers. Though good beers can be made utilizing the top-off method. Just make sure your recipe makes sense and be careful not to introduce bacteria.
5 gallons of beer takes just as much time to make as 2.5 gallons, so yeah, it's worth it.
I wouldn't recommend using a lid unless it's halfway covering the top of the kettle to slow down boil-off (or if you're only using it to quicken the boil start at the beginning of the brewing process). You really want to achieve a vigorous boil for many reasons while also allowing the wort to reduce for flavor and recipe purposes. IMO, it's better to concentrate the wort via boiloff from a full volume state vs. starting with an already concentrated wort and then watering it down. They are not equivalent flavorwise even though you may hit the same OG in the end. The partial boil beer will also be darker in color in most cases.
Takuie is correct. You'll want to reduce the hops called for in the first 30 minutes' worth of boil additions 20% by weight to compensate for the increased efficiency from having the additional water.
Steve from Northern Brewer explained this very well below.
"The early additions spend enough time in the boil for reactions related to the formation of bittering compounds to precipitate. When you utilize more water these reactions are more easily facilitated, increasing the theoretical amount of potential bittering units imparted to the batch. This is part of what makes re-scaling commercial beer recipes a bit trickier than a simple ratio-based conversion. Later hop additions that are meant for flavor or aroma contributions don't impart those levels of hop flavor via the same reactions as bittering hops; theirs is more akin to steeping of tea leaves in boiling water for flavor or aroma, just the same is it is for beer."
He also pointed out that you will always get a more natural flavor for the malt backbone by using more water, so that tells me the flavor will be better in the end!
I've been brewing a lot of DIPAs and have been boiling as large a volume as I can. Next purchase will be an 11 gallon kettle, so I can do full boils. Right now, the most I can manage is about 4.5 gallons in a 5 gallon kettle. Never have any boilovers, but I am mindful of my wort and my gas stove is pretty sensitive.
There is no way hop utilization is where it needs to be for hop bomb IPAs, with a partial boil. Even with beers that don't push the hop-bitterness-envelope, you could cut back on your hops to achieve the same bitterness.
I'm still brewing partial mashes + extracts and I use a 7.5gal kettle. I usually start with about 4.25gal of water. At the end of the boil, I'm usually down to around 4gals. Then I top off with 1gal of cold tap water in my fermenter to get the wort to pitching temp.