This question is severely early considering the state of my 914 but I like to think things through (too much if you ask my family).
I have a 1971 914-4 that needs some rust repair and body work.
I intend for it to be primarily a track car but one that can be driven on the street. I always thought 7's and 8's were a good look on a 914 so I was planning to widen the rear fender some. I have some GT Racing rear quarter panels that say they will allow 12" wheels. I was not looking to go this wide but now that I have these I am considering using them. I like the aggresive look of a wide, low 914. If I had a big HP motor then this might be an easier call but I don't.
I have a very nice 2.0L, built to meet the old PCA Production class (BP) rules. I don't have any HP numbers for this motor but there are no big mods allowed for production class, just Euro pistons.

How wide can I go with flares before I don't have enough HP to push them thru the air?

Jim

To me, there is nothing worst than a gutless engine in a tricked out ride and nothing meaner than a beefed up engine in a stock looking car. When I took a turn sideways last summer and side swiped a median, I wished I had some 15" tires in the rear.

I think that you don't have enough HP to turn that big of a set of wheels and tires. It will hurt more than it will help, IMO. With a 2.0 I wouldn't want any more than 8's & 9's. I've been running 7's & 8's for years and I've done well like that, with my 2056 I think I'd be happier with 8's & 9's but I haven't been affording new wheels.

I had 7 & 8's with a stock 2.0 /4. It cornered like it was on rails. No power out of the hole, but, when you can carry a lot of speed into a corner it usually isn't a problem.

Too wide? What's that?
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Mark D.