I have $300 to reinvest back into my site, what should I spend it on?

Hmmm...sounds like you don't know what you need, so I will try and keep this as simple as possible. It looks to me that the only real actionable item is your mailing list, so I will assume that is where you want people to be/do.

If you have plenty of people signing up, but don't have people converting into paying customers afterwords, you have a value problem...spend your time getting more inline with your targets needs/wants/triggers, and matching them with a solution. Perhaps spend some money on the products in your market so you can get a first-hand feel for them.

If you have plenty of people landing on your page, but not signing up, you have a conversion problem...think about hiring a designer and/or copywriter to discuss your conversion goals and site plan, and make it happen.

If you don't have enough people coming in, you have a traffic problem, and you should spend your money and time in that area.

That is the best way I can put it without knowing anything about the porn addiction market. Just a few guidlines to consider when doing a site goal analysis, where is the problem...that is what you spend your money/time on.

I think the previous post pretty much hit right to the point. You have to determine where you weak link is, and that pretty much boils down to either traffic and/or conversions (both on the opt-in and sales conversion via email). Once you have determined this, it'd be pretty clear where to focus your efforts on.