A spider silk bullet proof vest of resistance equivalent to Kevlar would be lighter and thinner. Changes needed to reduce the elasticity of spider silk would not significantly affect the weight - basically what you'd be doing is changing hydrogen bonds (which can slip and rebond readily allowing for elasticity) for covalent bonding which is stronger but once broken will not readily rebond. The molecular weight of the protein is very large and you'd be only changing a few bonds. In some cases you add bonds by removing molecules, which makes it lighter so the net change may be small.
Much the same for adding metals. This is not a metal coating for the silk strand, but adding a few metal ions (isolated metal atoms) to the protein to affect the hydrogen bonding interactions. The weight of the metal ions would be a very small fraction of the entire protein's weight, certainly less than 5%*1. A three times greater strength for such a small increase in weight would obviously give a much better strength/weight and toughness/weight ratio.
Many people in the high tech materials industry have been very eager to be able to get spider silk for bullet proof vests and similar applications and I'm quite sure they are very well aware of the qualities of both Kevlar and spider silk. If spider silk is not better, why do they want it? And why have so many companies been trying so hard to make it?
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*1 If that seems implausible to you, talk to someone who knows biochemistry. Proteins are made of strings of amino acids which are quite heavy (molecular weight) and, depending on the structure, one metal ion could affect the hydrogen bonding of up to 6 or even 12 functional groups. (And not every amino acid will have a functional group but will still be adding its weight.)
Again, this is NOT a metal coating, but a few scattered metal ions (i.e.: ISOLATED atoms of metal). It's not a metallic structure that increases the strength (the metal atoms don't even affect each other) but the effects that the metal ions have on the hydrogen bonding that increases the strength of the silk.
