Here's my feeling on the injectors for the L99. My car has 254,000 miles on it. The original injectors have never been out of it. Every once in a while, I run a tank or two of premium through it to clean it out, preferably when I am going on a longer trip. Works great. I stuck a borescope down it's throat a while back and the injectors look very nice down in there. So, that being said, I don't feel that new injectors or having mine cleaned would be of any benefit to me.
Now, as for the rest of your tune-up, so to speak. Remove water pump and replace with a new one. The input shaft seal will start to leak soon, if it isn't already, draining coolant onto the opti-spark. Change the cap and rotor on the opti. Get a Delphi cap and rotor kit. You will need a E-4 inverted torx socket to do it. Also, with the water pump, change the hoses(all of them), and t-fitting and restrictor. Drain all of the coolant and flush the block. Get down underneath and remove the knock sensors on both sides when you do this, as it is the only way to completely flush the block. Refill the system with new coolant and destilled water. Check condition of airfilter, and replace as necessary. Cold air kits are not a bad thing. I installed a K&N kit a long time ago, and it netted me an almost 2 mpg on the highway increase in milage. Change the PCV valve. It's on the driver's side of the intake manifold. Change the serpentine belt. Change trans fluid. Change Diff fluid. Grease chassis. Change oil, if due. Previously mentioned spark plugs. Wires, maybe? I changed mine the first time, but not the second time, 40K miles ago.
Thats my .02, and it has worked well for me, at least so far. I'm half way through my second tune up's service life so far and I'm still running great.
96 Caprice Classic
L99 w/ K&N Cold Air Kit, Home Base Delete w/ 1LE elbow, Impala SS springs and sway bars, MSD coil & wires, filled rear decklid courtesy of Dr. Sinister, 6-way Bonneville front buckets, Jegs catback dual exhaust, working on an Olds Bravada console.
263,000 Miles and counting -now sold 9/10/12