Connecting to Delta at LAS is pretty quick and straightforward.
Passport control is usually pretty quiet (at least when 155 arrives) and although I use Global Entry the main line for visitors is usually pretty quiet and shouldn't take more than a few minutes..
Once you've grabbed your bags from the carousel there's a virgin desk for you to recheck your luggage - again, it's usually quiet so should only take a couple of minutes.
After that just follow signs for flight connections and clear security into E gates - this will likely be the slowest part as it's usually busy and it isn't exactly a well oiled machine. TSA pre is pretty quick though, so if you have that it would help.
Finally, you'll need to get a train from E gates to D gates to catch your Delta flight. It's a really short connection though and trains are frequent, so you'll only need a few minutes.
As you can see from the above, even if you factor in worst case scenarios, you should be absolutely fine with 2.5 hours.
On a final note, assuming you're connecting from Upper to Delta first class, you still won't have access to the lounge. Don't worry though, you're not really missing much. If you have access via priority pass i find the lounge at the E gates to be slightly better and, more importantly, much quieter. The one at D gates is usually rammed and it can be hard to find a seat. If you're not intending to use a lounge then go straight to D gates, as the choice of shops and restaurants is much better.
Have a great trip!