I've flown a number of times, either direct from Ottawa or by way of Toronto. Both Airports have U.S. Customs pre-clearance on U.S.-bound flights, so DME works the same as a domestic flight once you arrive at MCO. I've never had any problems with DME taking my luggage. I usually pack any of the heavy clothes into out luggage once we're at the airport, so we're dressed appropriately for Florida and our bags are always in the room when we get back from the first park visit...
From the U.S. Customs and Border Protection web site, these are the airports with pre-clearance:
Canada
Calgary Preclearance
Calgary International Airport
P.O. Box 155
2000 Airport Road, Northeast
Calgary, Alberta, Canada T2E 6W5
Port Director
Phone: 403-221-1734
Edmonton Preclearance
Edmonton International Airport
P.O. Box 9830
Edmonton, Alberta, Canada T5J 2T2
Port Director
Phone: 780-890-4515
Halifax Preclearance
Halifax Robert L. Stanfield International Airport Level 2
1 Bell Boulevard
Enfield, Nova Scotia, Canada B2T 1K2
Port Director
902- 873-7780
Montreal Preclearance
Montreal Trudeau International Airport
975 Romeo Vachon Boulevard, North
Room 109
Dorval, Quebec, Canada H4Y 1H1
Port Director
Phone: 514-636-3859
Ottawa Preclearance
Ottawa MacDonald-Cartier International Airport
1000 Airport Road, Private
Gloucester, Ontario, Canada K1V 9B4
Port Director
Phone: 613-523-8120
Toronto Preclearance
Lester B. Pearson International Airport
P.O. Box 115
Toronto AMF
Toronto, Ontario, Canada L5P 1A2
Port Director
Phone: 905-676-2606
Vancouver Preclearance
Vancouver International Airport
International Terminal - Level 3
Room C3742.0
Richmond, British Columbia V7B 1Y7
Port Director
Phone: 604-278-7422
Victoria Preclearance
c/o Victoria Preclearance
139 West First Street
Room 204
Port Angeles, Washington 98362
Port Director
Phone: 250-384-1894
Winnipeg Preclearance
Winnipeg International Airport
2000 Wellington Avenue
P.O. Box 15
Winnipeg, Manitoba, Canada R3H 1C1
Port Director
Phone: 204-783-2062