What are your thoughts on ambitious projects to really show off your voice vs realistically-might-be-bought-from-a-first-time-writer?
If the initial goal is really to be noticed, surely a big budget period epic will serve your writing better if that's what you really want to write, rather than coming up with a low budget containable thriller or something for the sake of it. Or would the big budget period epic risk making the new writer look as though they don't understand budgets/the market and therefore unprofessional?
When I see a big-budget period epic spec from a first-timer, usually only one thought passes into my head: "Ugh. This will be a waste of time that can only result in a PASS." It's pretty much as you describe in your latter sentence.
The market for period pieces? Dead. D-E-A-D. Unless you're hired on assignment I wouldn't write one because it will probably be a waste of your time.
Writers are typecast as much as actors are. If you submit a writing sample that's some 16th Century epic drama, it's probably not going to do much for showing how you might write a thriller, or a romantic comedy, or a horror film. As a writing sample, all it really shows is how well you write that sort of period drama.
On the outside chance that the writing is superb - and I'm talking "can't-put-this-down-I-have-ever-reason-to-PASS-and-move-onto-something-marketable-but-I-can't-stop-reading" superb - yes, you might be invited to submit another spec script
But guess what? All that means is that the other spec you send them is going to have to be more in the vein of what they want. And honestly, if you had something that they were more likely to respond to on its own, why on Earth didn't you submit that first?
Sci-Fi is another genre that's risky to spec, but at least there's more of a market for sci-fi than for period epics. If my two specs were 2012 and Robin Hood, I'd lead with 2012. I know that I personally would probably be more likely to give a recommendation of "Let's see what else they've got" if their submission was a marketable genre (if not for a newbie) than if it wasn't much of a viable project at all.
Ambitious projects are only worth writing if you've really got the goods to pull them off. Most of the time, a writer will need to have been around the block for a while before they've reached that level.
For some other thoughts in this vein, I'll direct you to these two posts:
Everyone starts somewhere, so don't insist on being pretentious right out of the gate.
Everyone starts somewhere - even Undressed has distinguished alumni.