Altering Hops ratios

If I were to want to increase the amount hops added for flavoring by say 1/4, how much would that impact flavor and aroma?

If you are talking about 5 gal. batch (or 10gal. for that matter) a .25 oz. increase will be negligible. I would start with at least .5 oz. for a noticable difference.

Where is it starting? If the aroma add was originally 1/4 ounce, you've doubled it. If it was 2 ounces, you won't notice the change.

I am more concerned with it raising the amount of bitterness, or will that not be affected

A .25 oz. at the end of the boil would be insignificant in terms of ibu's. Even a high alpha acid hop at .25 oz. for 15mins. you wouldn't notice.
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Let me re-phrase that last statement.
If you added 1/4 oz. of hops with an alpha acid % of 12 for 15 minutes to a beer with a gravity of 1.050, it would add approx. 5 ibu's. This could impact the detectable bitterness of your beer, but I'm not that sensitive. However, if the gravity of the beer you are brewing is relatively low, like 1.035, then you may want to consider the potential impact on the final product.
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