Poor understanding of properties of materials result in wasted money at best, or no significant improvement.
The metal stretch of a bolt (strain) is related to the stiffness of the material (Young's modulus), applied force, cross sectional area and length - the latter because strain is expressed as a percentage of the total nominal length). If you are dealing with a ferrous-based material, the Young's modulus doesn't vary outside about 210 GPa +/- 5%, regardless whether we're talking plain carbon steel, alloy steel, tool steel or some other exotic grade of steel, therefore not much to be gained there. Ultimate tensile strength (PSI or MPa at breakage) is rather useless in the discussion of bolts, but yield strength is more relevant. The higher the yield strength, the more stress you can apply (and the more strain/stretch by extension) before it stretches permanently longer than the original, unloaded state. But in and of itself, a higher strength bolt does not increase the degree of stiffness or resistance from stretching whatsoever.
SImply installing higher "strength" fasteners of any kind - bolts or studs - with all else being equal (diameter, and torque) will give you a goose-egg zero additional clamping force.
You need either to increase the bolt diameter, along with increasing the tightening torque, but then by doing the latter you can damage the aluminum head.
Installing higher strength fasteners and seeking more clamping force (within what the cylinder head can withstand), while not a bad thing in and of themselves, are very poor Band-Aids for the root of the issue, which is the need to manage peak cylinder pressures (PCPs).