The Drop-in's add about 5-8 hp (see thread below) with a little more if you decide to add HPS Tubes.
K&N Drop-In Filters
My new HPS post MAF tubes installed
Over-oiling with the K&N's is a non-issue as long as you follow the re-oiling instructions.
You can reach the over the 300 rwhp mark with a CBE and CAI, maybe a little more with an Uprev Tune. Every car is different and every dyno is different, so your gains should be based on baseline readings anyways.