That's the great irony with The War of 1812 - for all the grandstanding from Americans, Canadians and Brits about it, we all tend to forget that all sides waged the war fairly halfheartedly and incompetently.
I repeat my point from before though as far as gains - the British might cede the Ontario Peninsula to the USA, as they would have even been willing to cede it during the Revolutionary War.
The Northern Shore of the great lakes may be ceded with it. Not much point in keeping it without Ontario.
The big one though remains the West - the British lose nothing in ceding it at this point, and they may be willing to let the Americans have the Prairie and Oregon if they agree to let the Brits keep Ontario/Quebec/etc.
Either way, I expect the British to get a hefty cash settlement as part of the treaty - the USA gets more land to settle, the Brits offhand indefensible and mostly unsettled land, along with I assume enough money to make a dent in paying for the Napoleonic Wars, and both sides come away getting to call it a "victory".
The British might've ceded Ontario peninsula to the US? That's interesting. Where have you read that one?
As a possible scenario here:
-US wins one or more clear battles in or around Canada
-US gains west of the Lake of the Woods, but agrees to pay HBC ₤300,000 for it, and makes a sweet deal to the British for the lumber and furs. This will help the British pay down their debts, employ Americans, and compensate the British for the lost land. A future treaty clarifies the ambiguous wording which essentially yields Manitoba and everything north and west of it to the US. Thus the US gains 3-5 states plus one or more territories out of it.
-maybe the US gets an island? Bermuda? Bahamas?
-Canada still exists, begins forming its own identity like OTL, just with the eastern part of Canada: Ontario (peninsula up to French River/Lake Nipissing), Manitoulin (north of this Ontario, up to the 49th parallel, west to our Marathon, Ontario), Opasquia Territory (west of 86°22' W to
95°9′12.2″W and north to Nelson River, and having a shore along Hudson Bay), Quebec (northern border is 49° N to Lac St Jean, into the Saguenay river, and thence into St Lawrence, and including OTL southern Quebec; western border is OTL), New Brunswick, Prince Edward Island, and Nova Scotia are OTL; Newfoundland is its own colony as is Labrador. East Quebec would be a line from the northernmost point of Lac St Jean due north, an all the land east thereof not in Labrador. The remaining land is Hudson Bay Territory. Eleven subdivisions - 9 provinces, 2 territories. Anyone up for a map?