[elcius]

lua has no problem with very large tables, just make an itemid to setid lookup table.

local setName =  {
[1] = {  name = "Mystic's Regalia (Recolor)", items = {14090, 26008, 14094}, },
-- [2......1778]
[1779] = {  name = "Imperial Plate", items = {31436, 30002, 12424, 12425, 12422, 12427, 12429}, },
}
 
-- make lookup table, ideally this would be pre-made
local itemSet = {};
for i,set in pairs(setName) do
    for j, id in pairs(set['items']) do
        itemSet[id] = i;
    end
end
 
function addline_gametip()
    local itemName,itemLink = GameTooltip:GetItem();
    if not itemLink then return end
    
    local itemId = itemLink:match('item:(%d+)');
    local setIndex = itemSet[tonumber(itemId)];
    
    if setIndex and setName[setIndex] then
        GameTooltip:AddLine("Transmog Set: " .. setName[setIndex]['name']);
    end
 end
 
 
GameTooltip:HookScript("OnTooltipSetItem", addline_gametip);