The northern boundary is fuzzy whereas the other boundaries are well defined by the sea.
In the Old Statistical Accounts, there is the following definition of Cowal:
STRACHUR AND STRALACHLAN
The district of Cowal, containing 6 parishes, is a point of land, stretching from N. E. to S. W. between the river Clyde and Lochfine. The N. E. part of the district, which joins Perthshire, presents a very rugged and broken surface. The mountains become gradually lower and smoother, as you advance to the S. W. ; and towards the extremity, comparatively speaking, the land is low and evenly.
In that definition, to join Perthshire, it would need to include e.g. Beinn Ime.
Another definition from the same source:
DUNOON AND KILMUN
Presbytery.-Dunoon being the seat of the Presbytery bearing that name, it may be proper to remark, that the presbytery embraces the six parishes in the district of Cowal, viz. Dunoon and Kilmun, Inverchaolain, Kilmodan or Glendaruel, Kilfinan, Strachur and Stralachlan, and Lochgoilhead and Kilmorich or Cairndow
The Lochgoil-Head and Kilmorich entry does not however describe that parish with any mention of Cowal, which may indicate people there thought of Cowal being farther south.
What I can tell you is that in none of the sources I've looked at is the name 'Cowal peninsula' or 'Cowal Peninsula'. This adding of 'peninsula' to Scottish district names is a pox that seems to have arrived in the last 10 or so years. Now we have the Cowal peninsula, the Ardgoil Peninsula, the Kintyre peninsula, the Ardnamurchan peninsula. Yes they are geographically peninsulas, but so is Scotland. What's next? The Scotland peninsula?