Is there any altcoin with improvements in blockchain download time?
Your premise is wrong. The work required to download and validate the blockchain has nothing at all to do with the work required to mine it.

As far as the block hash goes, validating it requires just one hash while mining the block requires quintillions of hash calculations.

Validating the block also requires computing the hashes of the Merkle tree but the main work is validating ECDSA signatures. Your method is not going to help with that.

(On a meta note, it's rude to offer "improvements" when you don't understand well enough what currently exists.)