Not quite Imperial Stout aging question

I brewed an Imperial Stout yesterday and the OG ended up quite a bit lower than I expected. This was mainly because my final volume was 6 gal instead of 5 gal. I thought I would lose more water during the boil. I think from now on I need to plan on losing 0.5-0.75 gal during the boil and no more.

So instead of 1.088, I ended up at 1.072. This falls slightly out of the RIP range of 1.075-1.115.

My initial plan was to transfer to secondary after about 2 weeks or 1 month and then let it sit in the secondary for at least 6 months before bottling. Do you still think this amount of aging is necessary?

I would probably leave it in primary for the 2-4 weeks like you said. If you can drop the yeast enough in primary, though, you should be able to just secondary in bottles. Personally I doubt you'd actually need 6 months with a 1.072 beer (or a 1.088 for that matter, as long as you had a good fermentation (pitching rate was good/etc)). I'd say you need to age it until it tastes good. You'll have an idea when you taste it after 2-4 weeks how long you need to age it.

Well my airlock was borderline clogged, so I rushed to Home Depot to get 1 in. hoseline for a blowoff tube. When I pulled the stopper - SPEWWWW!!! So, I guess my yeast starter was healthy. Never seen this aggressive of a fermentation. I think from now on I'll use the blowoff on the primary and airlock on the secondary.

Problem is if I bottle it...I'll drink it.

Out of curiosity did you use White Labs Irish Ale yeast? First time I made a stout with that strain the same basic thing happened....this strain has been incredibly aggressive for me.

Nope. Wyeast 1728 Scottish Ale.

In the future, if you know you are going to have more than the desired final volume, and your OG is going to suffer as a result, just boil longer. Longer boil = more boiloff = lower volume = higher OG.