Here is a quote from the HP5+ data sheet on the Ilford website: http://www.ilford.com/html/us_english/pdf/HP5_Plus.pdf(posted 8253 days ago)"Cameras with through-the-lens metering will usually adjust the exposure automatically when using filters. With some automatic exposure cameras, the correction given for deep red and orange filters can produce negatives under exposed by as much as 1 1/2 stops."
I would run some tests using a gray card (or something in the middle gray area) to see what the real difference is with and without the filter. This will require you to take one shot without the filter and multiple shots (with varying exposure adjustments) with the filter to see which adjustment needs to be made.
There will likely be a difference depending on the nature of the light. In some areas (such as Colorado where I live) there is an abundance of blue light, especially in shadows on a sunny day. This requires me to make a larger adjustment than may be required in some other locations (like Rochester NY or anywhere in the UK).
As far as under/over development is concerned: if that is the problem it is probably underdevelopment, but I would work on the exposure first.