I have not noticed any porters at Penn Station ever to help with bags. However, that could be because I wasn't looking for the service. The best thing you could do is call Penn Station itself on a help or customer service number to see if there is some type of porter service to help with your bags. Maybe you could arrange something. But you need to call them to find out.
Getting a taxi would be no problem in Manhattan, especially around Penn Station. There are cabs everywhere! If there is no traffic, the ride to JFK should take you 30 - 45 minutes.
At what time are you going to make your trip?
The price depends on that as well. If you are trying to go during rush hour, the meter is going to be running. The taxis charge a metered rate, so price will vary. You should avoid rush hour. Not only for the price, but so you won't miss your flight.
One last thing, be careful of the unlicensed livery cabs. They are scammers. They use town cars and they will charge you double the price for a taxi. There's no need to use them. Take a taxi! Hope this helps!