First of all, the flapper has NOTHING to do with the huge dip in power from 4-5k. The flapper has to do with the little tiny dip around 5.5k, since this is when it closes.

The 4-5k dip is a result of the stock slip ons and is remedied by a high quality aftermarket set.

Secondly, it is much better to change the oil INSTEAD of the filter (if only choosing one) since -

a) the oil tends to break down with use and frankly just gets dirty, and

b) a dirty filter actually cleans BETTER than a brand new filter. Ex: an engine will loose performance with a dirty air filter because the filter is filtering too well.

Thank you.