machined aluminum should be significantly stronger as it is melted and extuded, making all the inconsistancies and air in the metal exit. cast metal is generally much less stable. a bend in cast is catastrophic, a bend in Machined is repairable.
As TFOX said, do not mistake castings for forgings. Very, very different processes.
Unless you are a machinist who cuts a lot of aluminum, you'd have no idea how many internal grain stesses are built up inside a billet of aluminum by the manufacturing process. You wouldn't believe how easy it is to bend billet stock aluminum. You wouldn't believe how much the part will contract and expand with differences in temperature. (Ever set your peep in hot weather, but the first cold snapthat comes along yourpeep doesn't line up with your eye? There's the termperature effect on a machinedaluminum riser.)Parts cut from billet are actually not all that strong,because the billet's internal grain structure is cut through as the part is being produced.
Forging, on the other hand, compresses the internal grain structure. Forging forces the internal grain to mirror the external shape of the part itself so it is far stronger than billet.Parts made by forging arefar more stable to temperature fluctuations than those machined from billet. But since the aluminum is forcibly condensed, it also makes the part heavier than an identical one machined from billet.
In my job as an aerospace machinist, I couldn't tell how many tons of aluminum billet I cut into aircraft/missle/satelite parts. Close tolerance parts had to be machined ina specific temperature controlled environment because billet is so sensitive to temperature changes.I also know when the engineersreally needed enhanced strength at lower cost for a particular part, they had them forged at a foundry.
Forging technology is remarkable. They can forge a part within very close tolerances where very little machining needs to be done on them. I'd bet now, 19 years since I last turned a crank on a milling machine, the technology is even more refined.
Really, I have always thought the machined riser mania was misguided and fueled by the ill informed. In some ways, particularly temperature stability,I even think the lowly cast riser is preferable to machined.