Valve springs are only part of valve float.
The other is lifter pump up. If you have hydraulic lifters (you do) at high RPM the oil either froths up or the oil pump is pumping too much oil. The lifter turns solid when it’s full of oil and drops when it’s empty. If it never empties, your valve stays put.
If the oil froths up and aerates, it will lock up the lifter altogether.
Part of the biggest problem with the SR is the ass-ness of the econo valvetrain.
With non adjustable rocker arms, there is no way to change preload on the hydraulic lifters. This is why you have to use shims.
Over time (like 60,000km) tolerances in the valvetrain will change. Your lifter preload is out the window. Add to that you have divorced rocker arms (one rocker controls two valves) and that they’re held on by magnets … under high RPM, the rocker arm will deflect a bit, essentially being hyper extended.
That’s when valvetrains start to come apart. As soon as a gap opens up, parts are going to break.
Stiffer valve springs are a start, but throwing them at a less-than-desireable valvetrain will probably cause more problems than anything else. Heavy springs add a lot of pressure to the valvetrain and will wear out parts prematurely. You also lose horsepower, especially on a DOHC.
What you have to determine is if you’re getting early float (valve springs) or valve float (pump up/springs)
Early float (valve bounce) is when you know you need better valve springs. What happens is the valve actually floats because the spring is smashed flat before it’s supposed to. This usually happens way before high RPM, hence why it’s called “early float”. The valve actually bounces up and down.
Valve float is not something to take lightly in an interference motor. As well, because it will happen during a compression stroke, you will have terrible overlap. Bad for truuuuuubooooooooooo.