There are no adverse consequences by using the 1.5 rocker. it is easier on your springs than the 1.6's so if thats what your worried about then dont. Because you arent sure what your springs are, i woudl definately run the 1.5's for insurance because they will eb easier on your valvetrain than the 1.6's. with the 306 cam and stock heads and stock bottom end, you arent gaining much from 1.6's, maybe 5 rwhp. ported heads and 306 cam might give you 10 additional hp with the use of 1.6 rr's, but thats it. 1.6 rr's arent the best bang for the buck mod to be honest. You can install them just as easy with the motor in the car as with the motor out. Valve covers are an easy removal(5 min) so dont let that allow you to make an irrational decision. With the 1.5 rr's, there is no need to take it easy, they are safer than the 1.6's, so dont worry about driving it easy, you can drive it just as hard if not harder than you would if you had the 1.6's on there beacuse you'll have less stress on the valvetrain. basically, what im trying to say is, 1.5's are safer than the 1.6's, will not hurt your car in ANY way shape or form, and will be easier on your current springs so dont hesitate to use them whiel you are waiting on your 1.6's. You wont notice a great deal of difference with the 1.6's anyways and you can beat the hell outta the car and feel safer with the 1.5's.