Boosting OG in extract recipes

There are some extract recipes I'd like to try but some of the are around 4 ~ 4.5 ABV which I already have plenty of, so I'd like to make something with a little more punch. I know that I can raise the OG by adding LME/DME/various sugars but I can't seem to find any info on what this does to the flavor. Any suggestions? How much does a partial mash add to fermentables?

If you are just looking to boost your alcohol content without contributing much flavor, you'll have better luck adding DME or some type of sugar. You'll want something that is highly fermentable so that it doesn't significantly affect body or flavor. It is fairly easy to determine exactly how much DME or sugar you would need for the OG you are looking for.

I agree that you will want to boost using DME rather than a partial mash. That having been said depending on how much of a boost you are looking for, it may have an effect on the IBU's. You can plug your recipe into calculator as it is, then add DME and see how it effects the rest of the numbers as you go. If you are looking to go from 4% to 6% it will most likely change the balance of the beer if you only adjust gravity.

Thanks for the feedback! I've been tinkering with BeerSmith which has a great "increase gravity" feature. You put in your recipe and tell it what gravity you are looking for and it adjusts your hops and malts quantities for you.