This is a perfectly fine technique if you don't want to do a double mash session to get it all grain. The only limitation with trying to go "imperial" using a large portion of extract (or doing it all extract if you aren't set up for all-grain brewing) is the fermentability of the extract.
Extract, by nature of how it is made, tends to have a limit to which it can be fermented. Its a function of trying to make the most versatile extract that they can. You may end up with an FG that is higher than you'd like in your final product, simply due to the amount of extract being used.
I can't give you #s because its highly dependent on the brand and amount of extract you end up using.
Here are my recommendations:
- Use the lightest colored DME you can. It tends to be lighter in color and fresher than most LME sources (unless you have a shop with high turnover). Focus then on using specialty grains for your desired flavor and color.
- The second recommendation would be to mash in fairly low for the all grain portion. Shooting for 148-149F for 90minutes will make for a highly fermentable wort.
- Lastly, after you figure out how much extract you think you want to use to get to your desired OG, I'd then substitute 10% of the gravity points from the extract you planned to use for the same # of gravity point with table sugar. Table sugar being almost 100% fermentable will help dry the beer out too.
I have heard of people sparging there grain beds with brewing water that contains a small percentage of their extract in it. Slowly fly sparging that through the grain bed at 160F ish tends to convert the less fermentable dextrans into simpler sugars on the way through the grains. Its sort of an off the wall variable that I don't have a lot of experience with yet, so I am not advocating it, but its something to keep in mind.