Planets in Proportions "only" have a 100% defence penalty (it is 200% in the stock game if memory serves), and a 10% offence bonus. However, weapon platforms can get higher bonuses than in the stock game: the PDC alone will give +40% (for PDC fire that is), you can get +15% at best via elite components, and there is a nice component giving a huge bonus to hit chances (Fire Control Center, ends at +90% offence or so).
If the planet is protected by adequate ECM, and considering all else is being equal, you will unfortunately need to go nuts with storms to offset all of these bonuses, making your own vessels virtually unable to hit anything, even if using the smallest mounts that give an accuracy bonus. And better be safe than sorry I guess.
