Thermolds are everybit as reliable and dependable as USGI 30 round magazines, if not more so. Provided they aren't new production Promag's, Thermolds are highly reliable and excellent magazines. Canadian Thermolds aren't made with Zytel and are less durable, but still very reliable and dependable. Wilson, NC Thermolds are the best and are some of the most reliable/durable STANAG magazines availible. Promag Thermolds aren't horrible, they are just Promag...and thats all I have against them. Thier Mini 14 Thermolds are every bit as reliable as my Wilson Mini 14 Thermolds, but I would not prefer Promag anything for a defensive weapon. I would load up the 10 Thermolds with M193 as opposed to storing the ammo on stripper clips. Strippers are a great way
to load magazines in a hurry, but having loaded magaziens is much more convienient. Load up your Thermolds and use your SA80 magazines for practice and load up your strippers with any new ammo you purchase. Thermolds are much better magazines than the steel SA80 magazines which have wild variances in manufacturing quality/specifications and can be less than reliable. The SA80 magazines were isolated as one of the biggest problems with the L85's operating malfunctions which is why they have been replaced by HK magazines and subsequently surplused. I have 8 SA80 magazines, all of which have been upgraded with new springs and "green" followers. I bought them during the ban, and despite some "fitting" they have been decent practice magazines, especially since the upgrades. My advice is as follows, load your Thermolds with the 300 rounds you have on stripper clips. Use your SA80 magazines for auxillery/range magazines to reduce wear/tear on primary defensive mags*USGI/Thermolds*. Buy a lot of Cproducts/D&H magazines as the prices are extremely low. While ordering your new magazines, buy D&H "takeout" springs and followers to upgrade your SA80 magazines. 44mag upgrades springs/followers and sells the stock components which are excellent. Installing these guts in your SA80 magazines will increase reliability and give you a few extra "defensive magazines".
I personally rely on Wilson Thermolds as defensive magazines. I load Thermolds with "specialty" ammo like bonded bullets or TAP so I can easily differentiate. You may consider doing the same and buying 300 rounds of a good HP/SP and loading them into the Thermolds and keeping the strippers for backup.