1.6 rockers increase your lift. Vortecs flow well above .500 lift so you should see some improvement. Only problem is stock heads only tolerate around .470 lift without doing some work around the valve seals/spring seats. If you're using the crate motor cam then you could have a problem with your seals on the exhaust side.
As stated Vortec heads won't tollerate much lift until they are modified quite a bit - I'm not sure they'll take much more than 430"/.440" stock and the 1.6's will just worsen the situation until they are played with a lot... $$$$
Roller rockers get rid of a lot of heat and friction and allow more rpms when used - but are only a crutch aside from that. Anytime the 1.6's make a lot more power (or any for that matter) over a 1.5 ratio is when they are making up for a camshaft that is usually to small or not appropriate for the application. If you are starting from scratch with a new motor combo. chosing the right cam initially (even when running 1.5's) is a better deal - if it would make more power with more rocker lift the cam choice probably wasn't right for that combo to begin with....Lots of guys only put 1.6's on the exhaust to free up power for a heavy vehicle - but it's only because they didn't get a split pattern cam with enough exhaust duration and /or lift in the first place!
I agree with Kevs10,that 1.6 rockers can be a bandaid.But there is more to it.Factory rocker ratios can be all over the place.So 1.6 or 1.5 rollers will be alot more accurate.1.6's are alot cheaper and easier to gain HP on a stock cam then changing the cam.1.6's accelerate the opening and closing of the valve giving you slightly more duration and getting the valve off the seat faster.They also, help the geometry between the valve stem and pushrod.
The down side is possible spring bind,and the chance you may have to elongate the pushrod slot in the head to keep them from rubbing.I've never worked with the Vortec heads I've heard about the spring problems,not sure about the pushrod slot.
It would be real nice if you could get a 1.6 to bolt on and rotate the motor by hand to check clearences before you buy.
I remember reading that you do need to elongate the pushrod slots for 1.6 rockers. They were selling a kit to do all the mods to the heads(seals, spring seats, and pushrod slots) for .500+ lift cams for under $200. Can't remember what mag. I saw it in. Those heads weren't made for ultra high lift cams anyways. The flow numbers start to peak around .400 lift with only a slight increase at .500 lift.
we did a vortec motor 2 months ago, any good machine shop can boss the spring seats for the size you need, we took a 97 vortec stripped the injection off it, broke motor down, had block checked, reassembled. Heads recieved a valve job, then spring seats were cut for bigger springs, used an Isky .510 lift cam and the LS1 springs at first, those springs only lasted a week, broke 3, sent heads back off again for more machine work, this time used springs rated for a .585 lift, the second go around with the heads ran $200 machining/springs. Edelbrock air gap intake with a Holley Classic 650 double pumper rounds motor out.....According to Scoggin-Dickey's website(www.sdpc2000.com) the stock heads/springs should be good to around .470 lift.
My original Vortec engine had around .525 total lift with the 1.6 rockers, I never had to elongate any holes, and pushrods weren't an issue. However, I found out real quick about breaking springs, and blowing seals.
I ended up having the retainers shaved down about .010 and that took care of it. I used a set of Crane LT1 springs with the inner spring removed, and never had any more problems.
Then I ditched it all and built a 383 stroker with Etec 200 heads.
Sometimes when you change to roller rockers, you have to go to tall valve covers as well...Just something else to check into/keep in mind...
I remember being so excited to finally put my rollers on, only to find that the tinware I had over them wouldn't work!!! Kinda sucks wanting to do burnouts but instead having to find tall covers on a Sunday...night....ROFL!!!