Can I reduce suction line down to 3/8" with an adapter?

Am installing a mini-split HV AC. Line set flare fittings call for 1/4" liquid line and 3/8" gas (suction) line, but was sent 1/4" and 1/2" respectively. So, I purchased two 3/8" to 1/2" adapters to use on ends of suction line. I have been told this might void the mfg warranty and reduce efficiency. I don't see why, as it is a larger than called for line and I am merely 'reducing' it to the called for 3/8", Shouldn't it only cost me the price of the freon to fill up the larger line? Also, the line set requires a 50' length instead of the usual 25' or 15'.